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This example shows how a property may evolve to be more and more strict over time by looking at a user name field. However, similar evolution may be applicable to other properties that start off with few restrictions and gradually become more constrained and formalized as the needs of the project evolve.

We’ll start with a fairly simple schema:

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type User {
    property name -> str;
}

At this stage we don’t think that this property needs to be unique or even required. Perhaps it’s only used as a screen name and not as a way of identifying users.

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$ 
edgedb migration create
did you create object type 'default::User'? [y,n,l,c,b,s,q,?]
> y
Created ./dbschema/migrations/00001.edgeql, id:
m14gwyorqqipfg7riexvbdq5dhgv7x6buqw2jaaulilcmywinmakzq
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$ 
edgedb migrate
Applied m14gwyorqqipfg7riexvbdq5dhgv7x6buqw2jaaulilcmywinmakzq
(00001.edgeql)

We’ve got our first migration to set up the schema. Now after using that for a little while we realize that we want to make name a required property. So we make the following change in the schema file:

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type User {
    required property name -> str;
}

Next we try to migrate:

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$ 
edgedb migration create
did you make property 'name' of object type 'default::User' required?
[y,n,l,c,b,s,q,?]
> y
Please specify an expression to populate existing objects in order to make
property 'name' of object type 'default::User' required:
fill_expr> 'change me'

Oh! That’s right, we can’t just make name required because there could be existing User objects without a name at all. So we need to provide some kind of placeholder value for those cases. We type 'change me' (although any other string would do, too). This is different from specifying a default value since it will be applied to existing objects, whereas the default applies to new ones. We then run edgedb migrate to apply the changes.

Next we realize that we actually want to make names unique, perhaps to avoid confusion or to use them as reliable human-readable identifiers (unlike id). We update the schema again:

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type User {
    required property name -> str {
        constraint exclusive;
    }
}

Now we proceed with the migration:

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$ 
edgedb migration create
did you create constraint 'std::exclusive' of property 'name'?
[y,n,l,c,b,s,q,?]
> y
Created ./dbschema/migrations/00003.edgeql, id:
m1dxs3xbk4f3vhmqh6mjzetojafddtwlphp5a3kfbfuyvupjafevya
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$ 
edgedb migrate
edgedb error: ConstraintViolationError: name violates exclusivity
constraint

Some objects must have the same name, so the migration can’t be applied. We have a couple of options for fixing this:

  1. Review the existing data and manually UPDATE the entries with duplicate names so that they are unique.

  2. Edit the migration to add an UPDATE which will de-duplicate name for any potential existing User objects.

The first option is good for situations where we want to signal to any other maintainer of a copy of this project that they need to make a decision about handling name duplicates in whatever way is appropriate to them without making an implicit decision once and for all.

Here we will go with the second option, which is good for situations where we know enough about the situation that we can make a decision now and never have to duplicate this effort for any other potential copies of our project.

We edit the last migration file 00003.edgeql:

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CREATE MIGRATION m1dxs3xbk4f3vhmqh6mjzetojafddtwlphp5a3kfbfuyvupjafevya
    ONTO m1ndhbxx7yudb2dv7zpypl2su2oygyjlggk3olryb5uszofrfml4uq
{
  WITH U := default::User
  UPDATE default::User
  FILTER U.name = .name AND U != default::User
  SET {
    # De-duplicate names by appending a random uuid.
    name := .name ++ '_' ++ <str>uuid_generate_v1mc()
  };

  ALTER TYPE default::User {
      ALTER PROPERTY name {
          CREATE CONSTRAINT std::exclusive;
      };
  };
};

And then we apply the migration:

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$ 
edgedb migrate
edgedb error: could not read migrations in ./dbschema/migrations: could not
read migration file ./dbschema/migrations/00003.edgeql: migration name
should be `m1t6slgcfne35vir2lcgnqkmaxsxylzvn2hanr6mijbj5esefsp7za` but `
m1dxs3xbk4f3vhmqh6mjzetojafddtwlphp5a3kfbfuyvupjafevya` is used instead.
Migration names are computed from the hash of the migration contents. To
proceed you must fix the statement to read as:
  CREATE MIGRATION m1t6slgcfne35vir2lcgnqkmaxsxylzvn2hanr6mijbj5esefsp7za
  ONTO ...
if this migration is not applied to any database. Alternatively, revert the
changes to the file.

The migration tool detected that we’ve altered the file and asks us to update the migration name (acting as a checksum) if this was deliberate. This is done as a precaution against accidental changes. Since we’ve done this on purpose, we can update the file and run edgedb migrate again.

Finally, we evolved our schema all the way from having an optional property name all the way to making it both required and exclusive. We’ve worked with the EdgeDB migration tools to iron out the kinks throughout the migration process. At this point we take a quick look at the way duplicate User objects were resolved to decide whether we need to do anything more. We can use re_test() to find names that look like they are ending in a UUID:

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db> 
... 
... 
select User { name }
filter
    re_test('.* [a-z0-9]{8}(-[a-z0-9]{4}){3}-[a-z0-9]{12}$', .name);
{
  default::User {name: 'change me bc30d45a-2bcf-11ec-a6c2-6ff21f33a302'},
  default::User {name: 'change me bc30d8a6-2bcf-11ec-a6c2-4f739d559598'},
}

Looks like the only duplicates are the users that had no names originally and that never updated the 'change me' placeholders, so we can probably let them be for now. In hindsight, it may have been a good idea to use UUID-based names to populate the empty properties from the very beginning.

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