fix: use attribute source for identity fields

lib/migration_generator/migration_generator.ex

@@ -2145,6 +2145,16 @@ defmodule AshPostgres.MigrationGenerator do
     end)
     |> Enum.sort_by(& &1.name)
     |> Enum.map(&Map.take(&1, [:name, :keys]))
+    |> Enum.map(fn %{keys: keys} = identity ->
+      %{
+        identity
+        | keys:
+            Enum.map(keys, fn key ->
+              attribute = Ash.Resource.Info.attribute(resource, key)
+              attribute.source || attribute.name
+            end)
+      }
+    end)
     |> Enum.map(fn identity ->
       Map.put(
         identity,

test/migration_generator_test.exs

@@ -75,12 +75,14 @@ defmodule AshPostgres.MigrationGeneratorTest do
         identities do
           identity(:title, [:title])
           identity(:thing, [:title, :second_title])
+          identity(:thing_with_source, [:title, :title_with_source])
         end
 
         attributes do
           uuid_primary_key(:id)
           attribute(:title, :string)
           attribute(:second_title, :string)
+          attribute(:title_with_source, :string, source: :t_w_s)
         end
       end
 
@@ -132,6 +134,10 @@ defmodule AshPostgres.MigrationGeneratorTest do
       # the migration creates unique_indexes based on the identities of the resource
       assert file_contents =~
                ~S{create unique_index(:posts, [:title, :second_title], name: "posts_thing_index")}
+
+      # the migration creates unique_indexes using the `source` on the attributes of the identity on the resource
+      assert file_contents =~
+               ~S{create unique_index(:posts, [:title, :t_w_s], name: "posts_thing_with_source_index")}
     end
   end