docs: add Add primary key and timestamps types to your Ecto schema guide

Author: fahchen

guides/plugins/primary_key_and_timestamps.md

@@ -0,0 +1,56 @@
+# Add primary key and timestamps types to your Ecto schema
+
+
+## Implementation
+
+This plugin use `c:TypedStructor.Plugin.before_definition/2` callback to
+inject the primary key and timestamps fields to the type definition.
+
+```elixir
+defmodule MyApp.TypedStructor.Plugins.PrimaryKeyAndTimestamps do
+  use TypedStructor.Plugin
+
+  @impl TypedStructor.Plugin
+  defmacro before_definition(definition, _opts) do
+    quote do
+      Map.update!(unquote(definition), :fields, fn fields ->
+        # Assume that the primary key is an integer
+        primary_key = [name: :id, type: quote(do: integer()), enforce: true]
+
+        # Add two default timestamps
+        timestamps = [
+          [name: :inserted_at, type: quote(do: NaiveDateTime.t()), enforce: true],
+          [name: :updated_at, type: quote(do: NaiveDateTime.t()), enforce: true]
+        ]
+
+        [primary_key | fields] ++ timestamps
+      end)
+    end
+  end
+end
+```
+
+## Usage
+
+```elixir
+defmodule MyApp.User do
+  use TypedStructor
+  use Ecto.Schema
+
+  # disable struct creation or it will conflict with the Ecto schema
+  typed_structor define_struct: false do
+    # register the plugin
+    plugin MyApp.TypedStructor.Plugins.PrimaryKeyAndTimestamps
+
+    field :name, String.t()
+    field :age, integer(), enforce: true # There is always a non-nil value
+  end
+
+  schema "source" do
+    field :name, :string
+    field :age, :integer, default: 20
+
+    timestamps()
+  end
+end
+```

mix.exs

@@ -31,7 +31,8 @@ defmodule TypedStructor.MixProject do
           "guides/migrate_from_typed_struct.md",
 
           # plugins
-          "guides/plugins/type_only_on_ecto_schema.md"
+          "guides/plugins/type_only_on_ecto_schema.md",
+          "guides/plugins/primary_key_and_timestamps.md"
         ]
       ],
       package: [