Easy EdgeDB · Chapter 13

Meet the new Lucy

IntrospectionType Union Operator

This time it was too late to save Lucy, and she is dying. Suddenly she opens her eyes, but they look very strange. She looks at Arthur and says “Arthur! Oh, my love, I am so glad you have come! Kiss me!”

Arthur tries to kiss her, but Van Helsing grabs him and says “Don’t you dare!” It was not Lucy, but the vampire spirit inside her that was talking. Lucy soon dies, and Van Helsing puts a golden crucifix on her lips to stop her from moving (crucifixes have that power over vampires). Unfortunately, the nurse steals the crucifix to sell when nobody is looking and vampire Lucy is able to move again…

A few days later there is news about a lady who is stealing and biting children. The newspaper calls it the “Bloofer Lady”, because the young children try to call her the “Beautiful Lady” but can’t pronounce beautiful right.

Van Helsing decides that it’s time to tell the other people the truth about vampires, but Arthur doesn’t believe him and becomes angry that he would say crazy things about his wife. Van Helsing says, “Fine, you don’t believe me. Let’s go to the graveyard together tonight and see what happens. Maybe then you will.”

Looks like Lucy, an NPC, has become a MinorVampire. How should we show this in the database? Let’s look at the types again first.

Right now MinorVampire is nothing special, just a type that extends Person:

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type MinorVampire extending Person;

Fortunately for us, according to the book she is a new “type” of person. The old Lucy is gone, and this new Lucy is now one of the slaves linked to the Vampire object named Count Dracula. We can treat them as two separate objects.

So instead of trying to change the NPC type, we can just give MinorVampire an optional link to Person:

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type MinorVampire extending Person {
  former_self: Person;
}

The former_self link isn’t required because we don’t always know anything about people before they were made into vampires. For example, we don’t know anything about the three vampire women before Dracula found them so we can’t make an NPC type for them.

Another way to (informally) link them is to give the same date to last_appearance for an NPC and first_appearance for a MinorVampire. First we will update Lucy with her last_appearance:

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update Person filter .name = 'Lucy Westenra'
set {
  last_appearance := cal::to_local_date(1893, 9, 20)
};

After doing the migration, let’s practice a big insert to add Dracula along with all of the MinorVampire objects. We haven’t done much our existing objects so let’s just delete Vampire; and delete MinorVampire; and insert them all again.

Note the first line of the insert where we create a variable called lucy. We can then use that to bring in all the data to make her a MinorVampire, which is much more efficient than manually inserting all the information. It also includes her strength: we should add 5 to that, because vampires are generally stronger than humans.

We could give her the name ‘Lucy Westenra’ here because the name property is a delegated constraint from the Person type, but we’ll just call her Lucy now.

Here’s the insert:

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with lucy := assert_single(
    (select Person filter .name = 'Lucy Westenra')
)
insert Vampire {
  name := 'Count Dracula',
  age := 800,
  strength := 20,
  slaves := {
    (insert MinorVampire {
      name := 'Vampire Woman 1',
      strength := <int16>round(random() * 5) + 5
    }),
    (insert MinorVampire {
      name := 'Vampire Woman 2',
      strength := <int16>round(random() * 5) + 5
    }),
    (insert MinorVampire {
      name := 'Vampire Woman 3',
      strength := <int16>round(random() * 5) + 5
    }),
    (insert MinorVampire {
      name := 'Lucy',
      former_self := lucy,
      first_appearance := lucy.last_appearance,
      strength := lucy.strength + 5,
    }),
  },
  places_visited := (select Place filter .name in {'Romania', 'Castle Dracula'})
};

And thanks to the former_self link, it’s easy to find all the minor vampires that come from Person objects. Just filter by exists .former_self:

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select MinorVampire {
  name,
  strength,
  first_appearance,
} filter exists .former_self;

This gives us the following input (though strength will vary):

{
  default::MinorVampire {
    name: 'Lucy',
    strength: 9,
    first_appearance: <cal::local_date>'1893-09-20',
  },
}

Other filters such as filter .name in Person.name and .first_appearance in Person.last_appearance; are possible too but checking if the link exists is easiest. We could also switch to cal::local_datetime instead of cal::local_date to get the exact time down to the minute. But we won’t need to get that precise just yet.

Another operator related to types is |, which is used to combine them (similar to writing or). This query for example pulling up all Person types will return {true}:

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with lucy := (select Person filter .name like 'Lucy%'),
select lucy is NPC | MinorVampire | Vampire;

It returns true if the Person type selected is of type NPC, MinorVampire, or Vampire. Since Lucy the NPC and Lucy the MinorVampire match any of the three types, the return value is {true, true}.

One cool thing about the type union operator is that you can also add it to links in your schema. Let’s say for example there are other Vampire objects in the game, and one Vampire that is extremely powerful can control another less powerful vampire. Right now though a Vampire can only control a MinorVampire:

type Vampire extending Person {
  multi slaves: MinorVampire;
}

So to represent this change, you could just use | and add another type:

type Vampire extending Person {
  multi slaves: MinorVampire | Vampire;
}

We only have Count Dracula in our database as the main Vampire type so we won’t change our schema in this way, but keep the | operator in mind in case you need it.

We’ve decided to keep the existing NPC object for Lucy, because that Lucy will be in the game as an NPC until September 1893. Other PC objects could interact with her as an NPC up to this time, for example.

But what if we had chosen to delete her, what would have happened to the objects she is linked to? Or more realistically, what if all MinorVampire types connected to a Vampire should be deleted when the vampire dies? This is the way vampire physics works in Bram Stoker’s book: vampires drain people of their blood and turn them into minor vampires, who are only alive because the vampire is still controlling them. But when the master vampire dies, the souls of the minor vampires are finally set free and they disappear too.

We can begin thinking about these vampire physics in our game by learning about how to set a deletion policy. We’ll start with the keywords on target delete. This on target delete means “when the target is deleted”, or in other words “when the object that is linked to is deleted”. It goes inside {} after the link declaration. After this point there are some options four options to choose.

One deletion policy is restrict, and forbids you from deleting the target object. This is the default setting. In other words, anything you link to can’t be deleted unless you specify otherwise in the schema. So when you declare a type Vampire like this:

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type Vampire extending Person {
  multi slaves: MinorVampire;
}

It is as if you had written the following:

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type Vampire extending Person {
  multi slaves: MinorVampire {
    on target delete restrict;
  }
}

We can test this out right now with an attempt to delete one of the vampire women:

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delete MinorVampire filter .name = 'Vampire Woman 1';

Here is the error:

edgedb error: ConstraintViolationError: deletion of default::MinorVampire
(db56215a-268c-11ee-ab5e-6322976b513c) is prohibited by link target policy
  Detail: Object is still referenced in link slaves of default::Vampire
  (db561336-268c-11ee-ab5e-b338ce4886f8).

Another deletion policy is allow, and simply allows you to delete the target. Inside the Vampire type it would look like this, which would let us delete any MinorVampire linked to a Vampire object.

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type Vampire extending Person {
  multi slaves: MinorVampire {
    on target delete allow;
  }
}

A third deletion policy is called delete source, which deletes the source of a link when the target is deleted. Be careful with this deletion policy! You want to be absolutely certain when setting a policy that results in automatic deletions, because EdgeDB won’t let you know about the automatic deletions that happen as a result of another deletion query. And if you have an automatic deletion policy that leads to another type that has its own automatic deletion policy…you’ll end up with a cascade of deletions that maybe you didn’t expect to happen.

Now in our case, using on target delete delete source would delete Count Dracula if we deleted the MinorVampire (the target) called Vampire Woman 1. So this schema is the opposite of what we want!

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type Vampire extending Person {
  multi slaves: MinorVampire {
    on target delete delete source;
  }
}

Fortunately, to switch the target and source around we can just change the keyword target to source. This gives us a deletion policy of on source delete delete target, which looks like this:

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type Vampire extending Person {
  multi slaves: MinorVampire {
    on source delete delete target;
  }
}

Again, you want to be careful when setting a deletion policy like this one. But our database is small and controlled, so let’s add this on source delete delete target deletion policy to the Vampire type and do a migration.

Now let’s give this deletion policy a quick test. We’ll insert a Vampire named ‘Alucard’ who has bitten a man named Brian, and made him into a MinorVampire. We’ll insert both together:

insert Vampire {
  name := 'Alucard',
  slaves := (insert MinorVampire {name := "Brian"})
};

Now let’s make sure that both of them are in the database:

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select Vampire {**} filter .name = 'Alucard';

There they are! Selecting Alucard leads us to Brian as well thanks to the link.

{
  default::Vampire {
    strength: {},
    last_appearance: {},
    first_appearance: {},
    degrees: {},
    title: {},
    name: 'Alucard',
    pen_name: 'Alucard',
    conversational_name: 'Alucard',
    age: {},
    is_single: true,
    id: 5d3a42da-286a-11ee-9442-9fa367e8a4c0,
    places_visited: {},
    lovers: {},
    slaves: {
      default::MinorVampire {
        strength: {},
        last_appearance: {},
        first_appearance: {},
        degrees: {},
        title: {},
        name: 'Brian',
        pen_name: 'Brian',
        conversational_name: 'Brian',
        age: {},
        is_single: true,
        id: 5d3a4a96-286a-11ee-9442-c31d83f69190,
      },
    },
  },
}

Now if Alucard is killed, Brian should turn to dust and vanish as well:

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delete Vampire filter .name = 'Alucard';

And then let’s do a query to see if we can find Brian anywhere:

select Person filter .name = 'Brian';

The query returns {}. Thanks to the on source delete delete target deletion policy, Brian is gone too!

Deletion policies can be pretty tricky to get right so let’s put together another concrete example of one in our schema and walk through it step by step.

PCs can join together as parties inside games to work together on a common goal. We could allow players of our game to create a party that can then be joined by anyone who is interested. To start, let’s make a simple Party type that PC can link to.

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type Party {
  name: str;
}

type PC extending Person {
  required class: Class;
  required created_at: datetime {
    default := datetime_of_statement();
  }
  multi party: Party; # New link here
}

Easy enough! Now let’s think about what the multi party: Party line means in practice. It has a default on target delete restrict placed on it, which means that we can’t delete any Party that is linked to by a PC. Let’s give this a try by adding a Party and two PC objects:

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insert Party { name := "Ye olde party" };
insert PC {
  name := "Talloon",
  class := Class.Merchant,
  party := (select Party filter .name = "Ye olde party")
};
insert PC {
  name := "Alena",
  class := Class.Rogue,
  party := (select Party filter .name = "Ye olde party")
};

And now any attempt to delete Party; will give this error:

edgedb error: ConstraintViolationError: deletion of default::Party
(86b32874-299c-11ee-8bd8-737485b849cd) is prohibited by link target policy
  Detail: Object is still referenced in link party of default::PC
  (9a6eaabe-299c-11ee-8bd8-c76e1f17a3f2).

We don’t want old Party objects to just sit around in our database when no PCs are using them anymore, so let’s set up a deletion policy to delete any Party when all PC objects linking to it are deleted.

There are two items to think about here. First of all, simply adding a on source delete delete target as below will not work. But let’s give it a try and see what happens. First change the PC type to have this deletion policy and do a migration:

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type PC extending Person {
  required class: Class;
  required created_at: datetime {
    default := datetime_of_statement();
  }
  multi party: Party {
    on source delete delete target;
  }
}

And then let’s try to delete both PC objects that link to the Party object:

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delete PC filter .name in { "Alena", "Talloon" };

We get an error!

edgedb error: ConstraintViolationError: deletion of default::Party
(86b32874-299c-11ee-8bd8-737485b849cd) is prohibited by link target policy
  Detail: Object is still referenced in link party of default::PC
  (a2bb7ce2-299c-11ee-8bd8-431f1e9112d6).

This is because EdgeDB is attempting to delete Party when we delete each PC object, but there is still an invisible on target delete restrict policy that prevents the Party object from being deleted. In other words, our query tries to delete the PC called Alena but can’t because the PC called Talloon still links to the Party object.

So let’s add an on target delete allow to the PC type to allow the linked to Party object to be deleted. This is closer to what we want, but not quite! But let’s do a migration and see what happens in this case.

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type PC extending Person {
  required class: Class;
  required created_at: datetime {
    default := datetime_of_statement();
  }
  multi party: Party {
    on source delete delete target;
    on target delete allow;
  }
}

Okay, now let’s delete Talloon.

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delete PC filter .name = "Talloon";

Success! Talloon is now deleted. But hold on a second…where did the Party go?

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select Party;

The query returns an empty set! The PC named Alena is still in the database but her Party has outright disappeared. This Party object was automatically deleted because that’s what we instructed EdgeDB to do: adding on source delete delete target means “delete the target every time any object linking to it is deleted”. That’s not what we want.

The solution here is to add two new keywords: if orphan. Here is the difference once if orphan is added:

  • on source delete delete target means “delete the target if any object linking to it is deleted”

  • on source delete delete target if orphan means “delete the target if the last object linking to it is deleted”.

That’s what we want! So now let’s change the PC type to add these two new words and do another migration:

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type PC extending Person {
  required class: Class;
  required created_at: datetime {
    default := datetime_of_statement();
  }
  multi party: Party {
    on source delete delete target if orphan;
    on target delete allow;
  }
}

Next we have a bit of work to insert the Party object again, link the existing PC named Alena to it, and then to insert the PC named Talloon again…

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insert Party { name := "Ye olde party" };
update PC filter .name = "Alena" 
  set { 
    party := (select Party filter .name = "Ye olde party") 
  };
insert PC { 
  name := "Talloon",
  class := Class.Merchant,
  party := (select Party filter .name = "Ye olde party") 
  };

And with that we are ready to delete our PC objects one at a time again. We’ll start with Talloon again:

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delete PC filter .name = "Talloon";

Alena is still linking to the Party object, so it should still be there. Let’s check:

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select Party { name };

It sure is!

{default::Party {name: 'Ye olde party'}}

And with just a single (orphan) link left, deleting Alena the PC should delete the Party object as well. Let’s try it:

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delete PC filter .name = "Alena";

Now if we try select Party; we will get an empty set, just as we hoped!

We are already familiar with one of the set operators in EdgeDB: union. This is used to join two sets together. So select MinorVampire.name union MinorVampire.name; will return the following:

{
  'Vampire Woman 1',
  'Vampire Woman 2',
  'Vampire Woman 3',
  'Vampire Woman 1',
  'Vampire Woman 2',
  'Vampire Woman 3',
}

The distinct keyword is used if we want to remove duplicate values in a set, and is easy: just change select to select distinct.

A quick example of a property that has a lot of duplicates is the strength property on the Person type. A quick select Person.strength; will show this. The output will vary because it comes from the random function, but it will look something like this:

{1, 1, 7, 8, 9, 9, 5, 3, 0, 0, 5, 10, 2, 0, 
2, 5, 3, 4, 0, 1, 4, 20, 5, 5, 5, 5, 5}

Change it to select distinct Person.strength; and now only the distinct values remain:

{0, 1, 2, 3, 4, 5, 10, 20}

Note that distinct works by item and doesn’t unpack or aggregate, so something like a set of arrays will check to see if the entire array is distinct or not, not each of the values inside. Thus using distinct on the following query won’t do anything!

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select distinct {[7, 8], [7, 8], [9]};

It will simply return the original {[7, 8], [9]}, and not {7, 8, 9}.

The next set operator is called intersect and returns all the items in one set that match any item in the other set. Let’s try this one out on the places_visited property on the Person type. First let’s look at the places visited by all our NPC and PC objects:

db> select PC.places_visited.name;
{'Buda-Pesth', 'Bistritz', 'Munich'}
db> select NPC.places_visited.name;
{'Romania', 'Castle Dracula', 'Buda-Pesth', 'Bistritz', 'London', 'Munich'}

Now let’s intersect them!

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select PC.places_visited.name intersect NPC.places_visited.name;

This will simply return {'Bistritz', 'Buda-Pesth', 'Munich'}.

What if we want to use intersect on some object types and give them a shape? Let’s try giving a shape to all these Place objects instead of just showing their name. Simply putting a {*} after an intersect might seem to be the right way to make this happen:

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select PC.places_visited intersect NPC.places_visited {*};

And indeed the query does work, but the output is a bit unexpected. The shape is gone!

{
  default::City {id: da23b4be-268c-11ee-ab5e-0bda71669b0c},
  default::City {id: da317478-268c-11ee-ab5e-8b58f378092b},
  default::City {id: da41f9ec-268c-11ee-ab5e-e3cd443a19f2},
}

The issue here is that set operators don’t preserve the original expression type, so they don’t preserve the shape of an expression either. There is in effect no shape for us to work with.

Fortunately, the solution here is fairly simple: we can use with to capture the result of a set operator, and then that will have a shape that we can work with. So a small change to our query will do the job:

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with common_locations := PC.places_visited intersect NPC.places_visited,
  select common_locations {*};

The result is what we hoped to see: not just the names of the places visited by both PC and NPC objects, but their properties too.

{
  default::City {
    id: da41f9ec-268c-11ee-ab5e-e3cd443a19f2,
    important_places: ['Golden Krone Hotel'],
    modern_name: 'Bistrița',
    name: 'Bistritz',
  },
  default::City {
    id: da23b4be-268c-11ee-ab5e-0bda71669b0c,
    important_places: {},
    modern_name: {},
    name: 'Munich',
  },
  default::City {
    id: da317478-268c-11ee-ab5e-8b58f378092b,
    important_places: {},
    modern_name: 'Budapest',
    name: 'Buda-Pesth',
  },
}

The last set operator to learn is called except, and it’s the opposite of intersect. While intersect returns items that are in the first set as well as the other, except returns items from the first set that are not shared with the second set.

The except operator is a good opportunity to demonstrate that order can matter when using a set operator. Our PC objects have been to three cities: {'Buda-Pesth', 'Bistritz', 'Munich'}. The NPC objects have been to more places: {'Romania', 'Castle Dracula', 'Buda-Pesth', 'Bistritz', 'London', 'Munich'}. What do you think will happen in the query below that uses except? Note the order in which it is done.

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select PC.places_visited.name except NPC.places_visited.name;

That’s right, the query simply returns a {} empty set. That’s because EdgeDB went through the three names returned by PC.places_visited.name as follows:

  • ‘Buda-Pesth’: Is this one inside NPC.places_visited.name? Yes. So don’t return it.

  • ‘Bistritz’: Same.

  • ‘Munich’: Same.

And then the query was done.

But if we reverse the query then it will return some names:

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select NPC.places_visited.name except PC.places_visited.name;

The result is {'Castle Dracula', 'London', 'Romania'}, because this time EdgeDB began with the names of the places visited by the NPC objects and found three names that didn’t have a matching value in the set of names returned by the PC objects.

In other words, you can sort of think of except as meaning minus.

We saw back in Chapter 8 that we can use __type__ to get object types in a query, and that __type__ always has a name property that shows us the type’s name (otherwise we will only see its uuid). In the same way that we can get all the names of Person objects with select Person.name, we can use __type__.name to get all the type names that extend the Person type:

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select Person.__type__.name;

The output shows us all the names of types attached to Person so far, namely the types that extend Person:

{
  'default::NPC',
  'default::Sailor',
  'default::MinorVampire',
  'default::Crewman',
  'default::Vampire',
  'default::PC',
}

On top of the name property, the two most useful fields inside __type__ are properties and links. (You can also just choose the field pointers which holds both properties and links together.) If we add these to the query the output will get quite long.

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select Person.__type__ {
   name,
   properties: {name},
   links: {name}
};

The output contains the name, properties and links of each and every type that extends Person:

{
  schema::ObjectType {
    name: 'default::NPC',
    properties: {
      schema::Property {name: 'strength'},
      schema::Property {name: 'last_appearance'},
      schema::Property {name: 'first_appearance'},
      schema::Property {name: 'degrees'},
      schema::Property {name: 'title'},
      schema::Property {name: 'name'},
      schema::Property {name: 'pen_name'},
      schema::Property {name: 'conversational_name'},
      schema::Property {name: 'is_single'},
      schema::Property {name: 'id'},
      schema::Property {name: 'age'},
    },
    links: {
      schema::Link {name: '__type__'},
      schema::Link {name: 'places_visited'},
      schema::Link {name: 'lovers'},
    },
  },
  schema::ObjectType {
    name: 'default::Sailor',
    # And so on...

For such type-related queries, EdgeDB has a keyword called introspect that is specialized for looking inside types. (Indeed, the word introspect itself means to “look inside”.) It’s a little bit similar to adding __type__ to a query, but is more focused and has certain abilities and uses that you can’t get by using __type__.

A good rule of thumb is that:

  • If you are doing a query on objects and want to add some type information on the object itself, adding __type__ lets you do this.

  • If you want to do a query exclusively on the type itself, go with introspect.

To do an introspect query, just wrap it in parentheses inside a select. Here is how we can use introspect to look inside the Person type:

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select (introspect Person) {
    name,
    pointers: {name}
};

Both the query and output are now quite clean:

{
  schema::ObjectType {
    name: 'default::Person',
    pointers: {
      schema::Link {name: '__type__'},
      schema::Property {name: 'id'},
      schema::Link {name: 'lovers'},
      schema::Property {name: 'is_single'},
      schema::Link {name: 'places_visited'},
      schema::Property {name: 'age'},
      schema::Property {name: 'name'},
      schema::Property {name: 'title'},
      schema::Property {name: 'conversational_name'},
      schema::Property {name: 'degrees'},
      schema::Property {name: 'pen_name'},
      schema::Property {name: 'first_appearance'},
      schema::Property {name: 'last_appearance'},
      schema::Property {name: 'strength'},
    },
  },
}

Using introspect also lets us look inside scalar types, which isn’t possible with __type__ (which only works on object types). So this query won’t work:

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select Class.__type__ {*};

But introspect will:

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select (introspect Class);

That query just returns a {schema::ScalarType {id: c7c181cc-268c-11ee-980e-a10f818aefc0}}. But ScalarType looks like an object type of its own that we can use the splat operator on! Let’s see what’s inside:

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select (introspect Class) {*};

There it is! Lots of info about our Class enum:

{
  schema::ScalarType {
    final: false,
    is_final: false,
    abstract: false,
    is_abstract: false,
    id: c7c181cc-268c-11ee-980e-a10f818aefc0,
    name: 'default::Class',
    internal: false,
    builtin: false,
    computed_fields: [],
    expr: {},
    from_alias: false,
    is_from_alias: false,
    inherited_fields: [],
    default: {},
    enum_values: ['Rogue', 'Mystic', 'Merchant'],
    arg_values: {},
  },
}

You can even introspect the most basic of EdgeDB’s scalar types:

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select ((introspect str), (introspect int64), (introspect int16));

The output for this query is actually pretty interesting. We can see that the ids for EdgeDB’s basic scalar types have been manually chosen instead of automatically generated.

{
  (
    schema::ScalarType {id: 00000000-0000-0000-0000-000000000101},
    schema::ScalarType {id: 00000000-0000-0000-0000-000000000105},
    schema::ScalarType {id: 00000000-0000-0000-0000-000000000103},
  ),
}

But the introspect keyword isn’t limited to doing queries on types for our own information and fun. EdgeDB has one type that requires you to introspect it whenever it gets used! Let’s take a look at that now.

We made a few inserts of Crewman objects a few chapters ago, in which we gave them each a number. To do that, we used the count() function to count the number of Crewman objects, to which we added one:

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with next_number := count(Crewman) + 1,
  insert Crewman {
  number := next_number
};

This gave us a sequence of numbers.

Well, it just so happens that EdgeDB has a type called std::sequence that has this incrementation built in. This type is defined as an “auto-incrementing sequence of int64”, so an int64 that starts at 1 and goes up every time you increment it.

A sequence is used as an abstract type for other type names to extend, which allows each one of these sequence types to increment independently of any others. Note the similarity to the enum syntax we are familiar with:

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scalar type SomeSequence extending sequence;
scalar type SomeEnumType extending enum<OptionOne, OptionTwo, OptionThree>;

And once it is defined, we can just stick it on our object types as a property.

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scalar type SomeSequenceNumber extending sequence;

type SomeType {
  # This wouldn't work
  # required number: sequence;

  # But this will
  required number: SomeSequenceNumber;
}

This sequence number could be useful for our PC objects, because players of our game might want us to delete their accounts. When that happens we will have to delete the PC object that the player used, so the data will be gone. That means that we can’t use count(PC) as a sequence number. If we did that, then the 51st player would have the number 51, but if a PC object was then deleted then the next one would also be number 51! A sequence is just right for us here.

Let’s experiment first. We’ll add a SomeSequenceNumber to our schema with the following, and do a migration:

Copy
scalar type SomeSequenceNumber extending sequence;

And now let’s play around with this sequence type a bit before we make a PCNumber to put on our PC type as a property. Basic sequence behavior is pretty simple: you increment them with the sequence_next() function, and reset them with sequence_reset(). But here’s an important point: inside these functions you need to specify the sequence type that we want to increment.

In other words, just typing sequence_next() won’t work because EdgeDB doesn’t know which sequence type we want to increment:

db> select sequence_next();
error: QueryError: function "sequence_next()" does not exist
  ┌─ <query>:1:8
  │
1 │ select sequence_next();
  │        ^^^^^^^^^^^^^^^ Did you want "std::sequence_next(seq: schema::ScalarType)"?

And typing SomeSequenceNumber won’t work either because SomeSequenceNumber isn’t an object type in our schema:

db> select sequence_next(SomeSequenceNumber);
error: InvalidReferenceError: object type or alias 'default::SomeSequenceNumber' does not exist
  ┌─ <query>:1:22
  │
1 │ select sequence_next(SomeSequenceNumber);
  │                      ^^^^^^^^ error

But did you notice that the function is expecting an argument of ScalarType? We saw this just above when we learned the introspect keyword. So let’s try replacing SomeSequenceNumber with introspect SomeSequenceNumber which returns a ScalarType:

db> select sequence_next(introspect SomeSequenceNumber);
{1}

Success! Just add introspect and the sequence_next() and sequence_reset() functions will know which sequence type to increment.

Now that we know how sequences work, let’s play around with this sequence number of ours for a bit. As you can see, it can be incremented or reset, but can’t be reset to anything less than 1.

db> select sequence_next(introspect SomeSequenceNumber);
{2}
db> select sequence_next(introspect SomeSequenceNumber);
{3}
db> select sequence_next(introspect SomeSequenceNumber);
{4}
db> select sequence_next(introspect SomeSequenceNumber);
{5}
db> select sequence_reset(introspect SomeSequenceNumber, 10);
{10}
db> select sequence_reset(introspect SomeSequenceNumber, 0);
edgedb error: NumericOutOfRangeError: setval: value 0 is out of bounds for 
sequence "6f7e322d-ff25-11ed-95e6-558fd8f3e188_sequence" (1..9223372036854775807)
db> select sequence_reset(introspect SomeSequenceNumber, 1);
{1}

Finally, let’s change the schema for our PC type to include this number. We could type sequence_next(introspect PCNumber) every time we insert a PC object, but it’s much easier just to set sequence_next(introspect PCNumber) in the schema as the default value. Delete SomeSequenceNumber from the schema if you like, and then change the schema around PC to look like the following and do a migration:

Copy
scalar type PCNumber extending sequence;

type PC extending Person {
  required class: Class;
  required created_at: datetime {
    default := datetime_of_statement()
  }
  required number: PCNumber {
    default := sequence_next(introspect PCNumber);
  }
}

And now let’s do a quick query to see if it worked:

Copy
select PC {
  name, 
  number,
  created_at
};

And with that, the sequence incrementing just works. That was easy! And thanks to the default value we provided, EdgeDB has even updated our existing object types with the number property.

{
  default::PC {
    name: 'Emil Sinclair',
    number: 1,
    created_at: <datetime>'2023-05-28T10:40:53.598763Z',
  },
  default::PC {
    name: 'Max Demian',
    number: 2,
    created_at: <datetime>'2023-05-30T01:13:28.022340Z',
  },
}

Here is all our code so far up to Chapter 13.

Practice Time
  1. How would you insert an NPC named ‘Mr. Swales’ who has visited the City called ‘York’, the Country called ‘England’, and the OtherPlace called ‘Whitby Abbey’? Try it in a single insert.

    Show answer
  2. How readable is the result of this introspect query?

    Copy
    select (introspect Ship) {
      name,
      properties,
      links
    };
    
    Show answer
  3. What would be the shortest way to see what links form the Vampire type?

    Show answer
  4. What do you think the output of select distinct {1, 2} + {1, 2}; will be?

    Hint: don’t forget the Cartesian multiplication.

    Show answer
  5. What do you think the output of select distinct {2, 2} + {2, 2}; will be?

    Show answer

Up next: An old friend returns.

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