Easy EdgeDB · Chapter 20

The final battle

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You made it to the final chapter, congratulations! Here’s the final scene from the last chapter, though we won’t spoil the final ending in case you now plan to read the original book:

Mina is almost a vampire now, and says she can feel Dracula all the time, no matter what hour of the day. Van Helsing arrives at Castle Dracula and Mina waits outside. Van Helsing then goes inside and destroys the vampire women and Dracula’s coffin.

At the same time the four other heroes (Jonathan Harker, Quincy Morris, Doctor Seward, Arthur Holmwood) are approaching from the south to try to catch up to Dracula. The sun is still up, so Dracula is inside his box protected from the sunlight on a wagon that his friends are taking to the castle as fast as they can. The sun is almost down, it is snowing, and our heroes need to hurry to catch him. A fight ensues, and the heroes drive back Dracula’s friends and finally reach the box. They pull the nails back and open it up, and see Dracula lying inside. Jonathan pulls out his knife. But just then the sun goes down and night begins. Dracula smiles and opens his eyes, and…

If you’re curious about the ending to this scene, just check out the book on Gutenberg and search for “the look of hate in them turned to triumph”.

We are sure that the vampire women have been destroyed, however, so we can do one final change by giving them a last_appearance. Van Helsing destroys them on November 5, so we will insert that date. But don’t forget to filter Lucy out - she’s the only MinorVampire that isn’t one of the three women at the castle.

Copy
update MinorVampire filter .name != 'Lucy'
set {
  last_appearance := <cal::local_date>'1893-11-05'
};

Depending on what happens in the last battle, we might have to do the same for Dracula or some of the heroes…

Here’s the schema and inserted data we have up to Chapter 20.

Now that you’ve made it through all 20 chapters, you should have a good understanding of the schema that we put together and how to work with it. Let’s take a look at it one more time from top to bottom. We’ll make sure that we fully understand it and think about which parts are good, and which need improvement, for an actual game.

First let’s start with the schema in general.

  • To migrate a schema, just use the edgedb migration tools. A simple edgedb migration create and edgedb migrate is all you need to migrate your schema to a new one.

  • module default {}: We only used one module (namespace) for our schema, but you can make more modules if you like. You can see the module when you use describe type as sdl (or as text).

Here’s an example with Person, which starts like this and shows us the module it’s located in:

abstract type default::Person

For a real game our schema would probably be a lot larger with various modules. We might see types in different modules like abstract type characters::Person and abstract type places::Place.

Our first type is called HasNameAndCoffins, which is abstract because we don’t want any actual objects of this type. Instead, it is extended by types like Place because every place in our game:

  1. has a name, and

  2. has a number of coffins (which is important because places without coffins are safer from vampires).

Copy
abstract type HasNameAndCoffins {
  required coffins: int16 {
    default := 0;
  }
  required name: str {
    delegated constraint exclusive;
    constraint max_len_value(30);
  }
}

We could have gone with int32, int64 or bigint for the coffins property but we probably won’t see that many coffins so int16 is fine.

Next is abstract type Person. This type is by far the largest, and does most of the work for all of our characters. Fortunately, all vampires used to be people and can have things like name and age, so they can extend from it too.

Copy
abstract type Person extending HasMoney {
  required name: str {
    delegated constraint exclusive;
  }
  multi places_visited: Place;
  multi lovers: Person;
  is_single := not exists .lovers;
  strength: int16;
  first_appearance: cal::local_date;
  last_appearance: cal::local_date;
  age: int16;
  title: str;
  degrees: array<str>;
  conversational_name := .title ++ ' ' 
    ++ .name if exists .title else .name;
  pen_name := .name ++ ', ' 
    ++ array_join(.degrees, ', ') if exists .degrees else .name;
}

exclusive is probably the most common constraint, which we use to make sure that each character has a unique name. This works because we already know all the names of all the NPC types. But if there is a chance of more than one “Jonathan Harker” or other character, we could use the built-in id property instead. This built-in id is generated automatically and is already exclusive.

Properties like conversational_name are computed properties. In our case, we added properties like first and last later on. It is tempting to remove name and only use first and last for every character, but the book has too many characters with names that wouldn’t fit this like Woman 2 and The innkeeper. In a standard database used to record the data for users of an app, we would certainly only use first and last and a field like email with constraint exclusive to make sure that all users are unique.

Every property has a type (like str, bigint, etc.). Computed properties have them too but we don’t need to tell EdgeDB the type because the computed expression itself tells the type. For example, pen_name takes .name which is a str and adds more strings, which will of course produce a str. The ++ used to join them together is called concatenation. On the other hand, with a computed property you do have to tell EdgeDB whether it is a property or a link.

The two links on the Person type are multi links. Without the keyword multi it will be a single link and can only link to a single other object. This means that you may need to add assert_single() when creating a link or it will give this error:

error: possibly more than one element returned by an expression
for a computed link 'former_self' declared as 'single'

On the other hand, backlinks have the opposite behavior: a backlink is a multi link by default, meaning that you have to write single otherwise.

For first_appearance and last_appearance we use cal::local_date because our game is only based in one part of Europe inside a certain period. For a modern user database we would prefer std::datetime because it is timezone aware and ISO8601 compliant.

So for databases with users around the world, datetime is usually the best choice. Then you can use a function like std::to_datetime() to turn five int64s, one float64 (for the seconds) and one str (for the timezone) into a datetime that is always returned as UTC:

db> select std::to_datetime(2020, 10, 12, 15, 35, 5.5, 'KST');
....... # October 12 2020, 3:35 pm and 5.5 seconds in Korea (KST = Korean Standard Time)
{<datetime>'2020-10-12T06:35:05.500Z'} # The return value is UTC, 6:35 (plus 5.5 seconds) in the morning

A similar abstract type to HasNameAndCoffins is this one:

Copy
abstract type HasNumber {
  required number: int16;
}

We only used this for the Crewman type, which only extends two abstract types and nothing else:

Copy
type Crewman extending HasNumber, Person {
  overloaded name: str {
    default := 'Crewman ' ++ <str>.number;
  }
}

This HasNumber type was used for the five Crewman objects, who don’t have names. Because their name property inherited from Person is required, we used their number property to give them each a name: Crewman 1, Crewman 2, and so on.

Copy
for n in {1, 2, 3, 4, 5}
  union (
    insert Crewman {
      number := n,
      first_appearance := cal::to_local_date(1893, 7, 6),
      last_appearance := cal::to_local_date(1893, 7, 16),
    }
  );

So even though HasNumber was rarely used, it could become useful later on. For types later in the game you could imagine this being used for townspeople or random NPCs: ‘Shopkeeper 2’, ‘Carriage Driver 12’, etc.

Our vampire types extend Person, while MinorVampire also has a single and non-required link to Person. This is because some characters begin as humans and are “reborn” as vampires. With this format, we can use the properties first_appearance and last_appearance from Person to have them appear in the game. And if one is turned into a MinorVampire, we can link the two.

Copy
type Vampire extending Person {
  multi slaves: MinorVampire {
    on source delete delete target;
    property combined_strength := (Vampire.strength + .strength) / 2;
  }
  army_strength := sum(.slaves@combined_strength);
}

type MinorVampire extending Person {
  former_self: Person;
  single master := assert_single(.<slaves[is Vampire]);
  master_name := .master.name;
};

With this format we can do a query like this one that pulls up all people who have turned into MinorVampires.

Copy
select Person {
  name,
  vampire_name := .<former_self[is MinorVampire].name
} filter exists .vampire_name;

In our case, that’s just Lucy: {default::NPC {name: 'Lucy Westenra', vampire_name: {'Lucy'}}} But if we wanted, we could extend the game back before the events of the book and link the vampire women to an NPC type. That would become their former_self.

The PC and Sailor types show two enums and one sequence that we used:

Copy
scalar type Rank extending enum<Captain, FirstMate, SecondMate, Cook>;

type Sailor extending Person {
  rank: Rank;
}

scalar type Class extending enum<Rogue, Mystic, Merchant>;

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);
  }
  multi party: Party {
    on source delete delete target if orphan;
    on target delete allow;
  }
  overloaded required name: str {
    constraint max_len_value(30);
  }
  last_updated: datetime {
    rewrite insert, update using (datetime_of_statement());
  }
  bonus_item: LotteryTicket {
    rewrite insert, update using (get_ticket());
  }
}

The PCNumber type has been quite useful, allowing us to keep track of how many PC objects have been created even if some of them get deleted later. If you end up adding and deleting a lot of PC objects then the following query will show pretty different numbers between the latest sequence number and the total number of PC objects:

Copy
with latest := (select <str>max(PC.number)),
select {'Total PCs created: ' ++ latest ++ ' Current PCs: ' ++ <str>count(PC) };

ShipVisit is one of our two “hackiest” (but most fun) types. We stole some of it from the Time type that we created earlier and later decided to turn into a single global object. Inside the ShipVisit type we have a clock property that is just a string, but gets used in this way:

  • by casting it into a cal::local_time to make the clock_time property,

  • by slicing its first two characters to get the hour property, which is just a string. This is only possible because we know that even single digit numbers like 1 need to be written with two digits: 01

  • by another computed property called vampires_are that is either Asleep or Awake depending on the hour property we just made, cast into an int16.

Copy
type ShipVisit {
  required ship: Ship;
  required place: Place;
  required date: cal::local_date;
  clock: str;
  clock_time := <cal::local_time>.clock;
  hour := .clock[0:2];
  vampires_are := SleepState.Asleep if <int16>.hour > 7 and <int16>.hour < 19
        else SleepState.Awake;
}

The NPC type is where we first saw the overloaded keyword, which lets us use properties, links, functions etc. in different ways than in the type’s parent type. Here we wanted to constrain age to 120 years, and to use the places_visited link in a different way than in Person by giving it London as the default.

Copy
type NPC extending Person {
  overloaded age: int16 {
    constraint max_value(120)
  }
}

Our Place type shows that you can extend as many times as you want. It’s an abstract type that extends another abstract type, and then gets extended for other types like City.

Copy
abstract type Place extending HasNameAndCoffins {
  modern_name: str;
  multi important_places: Landmark;
}

The important_places property used to be an <array<str>>, but in Chapter 16 we decided to create a Landmark type that would represent the smallest type of location that would appear in the game such as hotels, parks, universities, and so on. These locations are so small that they don’t need to track the number of coffins, because the number of coffins is relevant for larger spaces of land to help determine if the location can be easily terrorized by vampires or not.

Annotations: we used abstract annotation to add a new annotation:

Copy
abstract annotation warning;

This was necessary because by default a type can only have annotations called title, description, or deprecated. We only used annotations for fun for this one type, because nobody else is working on our database yet. But if we made a real database for a game with many people working on it, we would put annotations everywhere to make sure that they know how to use each type.

Our Lord type was only created to show how to use constraint expression on, which lets us make our own constraints:

Copy
type Lord extending Person {
  constraint expression on (contains(__subject__.name, 'Lord')) {
    errmessage := "All lords need \'Lord\' in their name";
  }
}

We might remove this in a real game, or maybe it would become type Lord extending PC so player characters could choose to be a lord, thief, detective, etc.

The Lord type uses the function std::contains() which returns true if the item we are searching for is inside the string, array, etc. It also uses __subject__ which refers to the type itself: __subject__.name means Person.name in this case. Here are some more examples from the documentation of using constraint expression on.

Another possible way to create a Lord is to do it this way, since Person has a property called title:

Copy
type Lord extending Person {
  constraint expression on (__subject__.title = 'Lord') {
    errmessage := "All lords need \'Lord\' in their title";
  }
}

This will depend on if we want to create Lord types with names just as a single string in .name, or by using .first, .last, .title etc. with a computed property to form the full name.

Our next types extending Place including Country and Region were looked at just last chapter, so we won’t review them here. But Castle is a bit unique:

Copy
type Castle extending Place {
  doors: array<int16>;
}

Back in Chapter 7, we used this in a query to see if Jonathan could break any of the doors and escape the castle. The idea was simple: Jonathan would try to open every door, and if he had more strength then any one of them then he could escape the castle.

Copy
with
  jonathan_strength := (select Person filter .name = 'Jonathan Harker').strength,
  doors := (select Castle filter .name = 'Castle Dracula').doors,
select jonathan_strength > min(array_unpack(doors));

However, later on we learned the any() function so let’s see how we could use it here. With any(), we could change the query to this:

Copy
with
  jonathan_strength := (select Person filter .name = 'Jonathan Harker').strength,
  doors := (select Castle filter .name = 'Castle Dracula').doors,
select any(array_unpack(doors) < jonathan_strength); # Only this part is different

And of course, we could also create a function to do the same now that we know how to write functions and how to use any(). Since we are filtering by name (‘Jonathan Harker’ and ‘Castle Dracula’), the function would also just take two strings and do the same query.

Also don’t forget that we needed std::array_unpack() because the function std::any() works on sets:

Copy
std::any(values: set of bool) -> bool

So this (a set) will work: select any({5, 6, 7} = 7);

But this (an array) will not: select any([5, 6, 7] = 7);

Our next type is BookExcerpt, which we imagined being useful for the developers creating the database. It would need a lot of inserts from each part of the book, with the text exactly as written. We chose to use index on for the date property, which will then be faster when we need to order by date. Remember to use indexes only where needed: they speed up queries that filter, order, and group, but make the database larger overall and slow down inserts and updates.

Copy
type BookExcerpt {
  required date: cal::local_datetime;
  required excerpt: str;
  index on (.date);
  required author: Person;
}

Next is our other fun and hacky type, Event.

Copy
type Event {
  required description: str;
  required start_time: cal::local_datetime;
  required end_time: cal::local_datetime;
  required multi place: Place;
  required multi people: Person;
  location: tuple<float64, float64>;
  index on (.location);
  ns_suffix := '_N_' if .location.0 > 0.0 else '_S_';
  ew_suffix := '_E' if .location.1 > 0.0 else '_W';
  url := get_url() 
    ++ <str>(math::abs(.location.0)) ++ .ns_suffix 
    ++ <str>(math::abs(.location.1)) ++ .ew_suffix;
}

This one is probably closest to an actual usable type for a real game. With start_time and end_time, place and people (plus url) we can properly arrange which characters are at which locations, and when. The description property makes it easy for users of the database to find events. It might contain something like 'The Demeter arrives at Whitby, crashing on the beach'.

And the output for the Event type is especially nice as JSON. You can imagine how useful this might be for our game setting:

{
  "id": "d80dde9c-fec9-11ed-9c27-bffd94675ea1",
  "description": "Dr. Seward gives Lucy garlic flowers to help her sleep. She falls asleep and the others leave the room.",
  "east": false,
  "location": [54.4858, 0.6206],
  "url": "https://geohack.toolforge.org/geohack.php?params=54.4858_N_0.6206_W54.4858_N_0.6206_W",
  "end_time": "1893-09-11T23:00:00",
  "start_time": "1893-09-11T18:00:00",
  "people": [
    {
      "strength": 4,
      "id": "f22b1910-fd08-11ed-ab09-9b95a39d5d69",
      "first_appearance": null,
      "last_appearance": "1893-09-20",
      "name": "Lucy Westenra",
      "age": null,
      "title": null,
      "conversational_name": "Lucy Westenra",
      "degrees": null,
      "pen_name": "Lucy Westenra"
    },
    {
      "strength": 2,
      "id": "dea9080a-fe8b-11ed-9759-f3313536553b",
      "first_appearance": null,
      "last_appearance": null,
      "name": "John Seward",
      "age": null,
      "title": null,
      "conversational_name": "John Seward",
      "degrees": null,
      "pen_name": "John Seward"
    },
    {
      "strength": 4,
      "id": "70bad6e0-fea2-11ed-97e2-ef3281250140",
      "first_appearance": null,
      "last_appearance": null,
      "name": "Abraham Van Helsing",
      "age": null,
      "title": "Dr.",
      "conversational_name": "Dr. Abraham Van Helsing",
      "degrees": "M.D., Ph. D. Lit., etc.",
      "pen_name": "Abraham Van Helsing, M.D., Ph. D. Lit., etc."
    }
  ],
  "place": [
    {
      "id": "d64af6d0-fec9-11ed-9c27-a3cf8be8d973",
      "important_places": null,
      "modern_name": null,
      "name": "Whitby",
      "coffins": 0
    }
  ],
  "excerpt": []
}

The last two types in our schema, Currency and Pound, were created just two chapters ago so they are still fresh in our mind. We won’t need to review them here.

You might want to take a look at or bookmark this page for reference during your projects. It contains the whole lexical structure of EdgeDB including items that are maybe too dry for a textbook like this one. This includes things like order of precedence for operators, all reserved keywords, which characters can be used in identifiers, and so on.

Help is always just a message away. The most active place to discuss EdgeDB and get help is our Discord server, while the discussion board on GitHub is another good place to start a conversation. You can also start an issue here on EdgeDB, or do the same for the Easy EdgeDB book on its dedicated GitHub repo.

Your knowledge of EdgeDB and EdgeQL should be pretty good at this point, and you may be looking to take your EdgeDB-powered product to the next step. With EdgeDB Cloud you can deploy your database instantly and connect from anywhere with near-zero configuration. Leave the infrastructure to us!

We hope you enjoyed learning EdgeDB through a story and are now familiar enough with it to implement it for your own projects. Ironically, if we wrote the book with enough detail to answer all your questions then we might never see you on our Discord! If that’s the case, then we wish you the best of luck with your projects. Let’s finish the book up with a poem from another book, the Lord of the Rings, on the endless possibilities of life.

The Road goes ever on and on Down from the door where it began. Now far ahead the Road has gone, And I must follow, if I can, Pursuing it with eager feet, Until it joins some larger way Where many paths and errands meet. And whither then? I cannot say.

See you, or not see you, however things turn out! Thanks again for reading.

We use ChatGPT with additional context from our documentation to answer your questions. Not all answers will be accurate. Please join our Discord if you need more help.