improvement: add migration_defaults for customizing default values

lib/data_layer.ex

@@ -293,6 +293,15 @@ defmodule AshPostgres.DataLayer do
         doc:
           "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: [
+        type: :keyword_list,
+        default: [],
+        doc: """
+        A keyword list of attribute names to the ecto migration default that should be used for that attribute. Only necessary if you need to override the defaults.
+
+        The string you use will be placed verbatim in the migration. Use fragments like `fragment(\\\\"now()\\\\")`, or for `nil`, use `\\\\"nil\\\\"`.
+        """
+      ],
       base_filter_sql: [
         type: :string,
         doc:

lib/data_layer/info.ex

@@ -25,7 +25,12 @@ defmodule AshPostgres.DataLayer.Info do
 
   @doc "A keyword list of customized migration types"
   def migration_types(resource) do
-    Extension.get_opt(resource, [:postgres], :migration_types, nil, true)
+    Extension.get_opt(resource, [:postgres], :migration_types, [])
+  end
+
+  @doc "A keyword list of customized migration defaults"
+  def migration_defaults(resource) do
+    Extension.get_opt(resource, [:postgres], :migration_defaults, [])
   end
 
   @doc "The configured check_constraints for a resource"

lib/migration_generator/migration_generator.ex

@@ -1909,6 +1909,7 @@ defmodule AshPostgres.MigrationGenerator do
                 destination_attribute_default:
                   default(
                     source_attribute,
+                    relationship.destination,
                     AshPostgres.DataLayer.Info.repo(relationship.destination)
                   ),
                 destination_attribute_generated: source_attribute.generated?,
@@ -2040,7 +2041,7 @@ defmodule AshPostgres.MigrationGenerator do
       &Map.take(&1, [:name, :source, :type, :default, :allow_nil?, :generated?, :primary_key?])
     )
     |> Enum.map(fn attribute ->
-      default = default(attribute, repo)
+      default = default(attribute, resource, repo)
 
       type =
         AshPostgres.DataLayer.Info.migration_types(resource)[attribute.name] ||
@@ -2210,7 +2211,8 @@ defmodule AshPostgres.MigrationGenerator do
 
   @uuid_functions [&Ash.UUID.generate/0, &Ecto.UUID.generate/0]
 
-  defp default(%{default: default}, _repo) when is_function(default) do
+  defp default(%{name: name, default: default}, resource, _repo) when is_function(default) do
+    configured_default(resource, name) ||
       cond do
         default in @uuid_functions ->
           ~S[fragment("gen_random_uuid()")]
@@ -2223,9 +2225,18 @@ defmodule AshPostgres.MigrationGenerator do
       end
   end
 
-  defp default(%{default: {_, _, _}}, _), do: "nil"
-  defp default(%{default: nil}, _), do: "nil"
-  defp default(%{default: value}, _), do: EctoMigrationDefault.to_default(value)
+  defp default(%{name: name, default: {_, _, _}}, resource, _),
+    do: configured_default(resource, name) || "nil"
+
+  defp default(%{name: name, default: nil}, resource, _),
+    do: configured_default(resource, name) || "nil"
+
+  defp default(%{name: name, default: value}, resource, _),
+    do: configured_default(resource, name) || EctoMigrationDefault.to_default(value)
+
+  defp configured_default(resource, attribute) do
+    AshPostgres.DataLayer.Info.migration_defaults(resource)[attribute]
+  end
 
   defp snapshot_to_binary(snapshot) do
     snapshot