docs: add value_to_postgres_default/3 documentation (#791)

Co-authored-by: Cayden Lords <caydenwlords@gmail.com>

Author: CaydenLords

documentation/dsls/DSL-AshPostgres.DataLayer.md

@@ -42,7 +42,7 @@ end
 | [`migrate?`](#postgres-migrate?){: #postgres-migrate? } | `boolean` | `true` | Whether or not to include this resource in the generated migrations with `mix ash.generate_migrations` |
 | [`storage_types`](#postgres-storage_types){: #postgres-storage_types } | `keyword` | `[]` | A keyword list of attribute names to the ecto type that should be used for that attribute. Only necessary if you need to override the defaults. |
 | [`migration_types`](#postgres-migration_types){: #postgres-migration_types } | `keyword` | `[]` | A keyword list of attribute names to the ecto migration type that should be used for that attribute. Only necessary if you need to override the defaults. |
-| [`migration_defaults`](#postgres-migration_defaults){: #postgres-migration_defaults } | `keyword` | `[]` | A keyword list of attribute names to the ecto migration default that should be used for that attribute. The string you use will be placed verbatim in the migration. Use fragments like `fragment(\\"now()\\")`, or for `nil`, use `\\"nil\\"`. |
+| [`migration_defaults`](#postgres-migration_defaults){: #postgres-migration_defaults } | `keyword` | `[]` | A keyword list of attribute names to the ecto migration default that should be used for that attribute. The string you use will be placed verbatim in the migration. Use fragments like `fragment(\\"now()\\")`, or for `nil`, use `\\"nil\\"`. For custom `Ash.Type` modules, see `c:AshPostgres.Type.value_to_postgres_default/3`. |
 | [`calculations_to_sql`](#postgres-calculations_to_sql){: #postgres-calculations_to_sql } | `keyword` |  | A keyword list of calculations and their SQL representation. Used when creating unique indexes for identities over calculations |
 | [`identity_wheres_to_sql`](#postgres-identity_wheres_to_sql){: #postgres-identity_wheres_to_sql } | `keyword` |  | A keyword list of identity names and the SQL representation of their `where` clause. See `AshPostgres.DataLayer.Info.identity_wheres_to_sql/1` for more details. |
 | [`base_filter_sql`](#postgres-base_filter_sql){: #postgres-base_filter_sql } | `String.t` |  | A raw sql version of the base_filter, e.g `representative = true`. Required if trying to create a unique constraint on a resource with a base_filter |

lib/data_layer.ex

@@ -308,7 +308,7 @@ defmodule AshPostgres.DataLayer do
         type: :keyword_list,
         default: [],
         doc: """
-        A keyword list of attribute names to the ecto migration default that should be used for that attribute. The string you use will be placed verbatim in the migration. Use fragments like `fragment(\\\\"now()\\\\")`, or for `nil`, use `\\\\"nil\\\\"`.
+        A keyword list of attribute names to the ecto migration default that should be used for that attribute. The string you use will be placed verbatim in the migration. Use fragments like `fragment(\\\\"now()\\\\")`, or for `nil`, use `\\\\"nil\\\\"`. For custom `Ash.Type` modules, see `c:AshPostgres.Type.value_to_postgres_default/3`.
         """
       ],
       calculations_to_sql: [

lib/type.ex

@@ -4,11 +4,19 @@
 
 defmodule AshPostgres.Type do
   @moduledoc """
-  Postgres specific callbacks for `Ash.Type`.
+  Postgres-specific callbacks for `Ash.Type`.
 
   Use this in addition to `Ash.Type`.
   """
 
+  @doc """
+  Return the text that will be placed into a migration as an attribute default.
+
+  The returned string is placed verbatim into generated migrations.
+
+  If this callback is not implemented, the migration generator falls back to
+  `EctoMigrationDefault`.
+  """
   @callback value_to_postgres_default(Ash.Type.t(), Ash.Type.constraints(), term) ::
               {:ok, String.t()} | :error