Add support for removing from schema as well

Change-Ecto-Schema-to-ExtendSchema.diff

    add support for removing from schema as well

diff --git a/apps/nectar/web/models/product.ex b/apps/nectar/web/models/product.ex
index 100e830..38174a7 100644
--- a/apps/nectar/web/models/product.ex
+++ b/apps/nectar/web/models/product.ex
@@ -2,7 +2,8 @@ defmodule Nectar.ModelExtension do
   defmacro __using__(_opts) do
     quote do
       Module.register_attribute(__MODULE__, :schema_changes, accumulate: true)
-      import Nectar.ModelExtension, only: [add_to_schema: 1]
+      Module.register_attribute(__MODULE__, :remove_schema_changes, accumulate: true)
+      import Nectar.ModelExtension, only: [add_to_schema: 1, remove_from_schema: 1]
       @before_compile Nectar.ModelExtension
     end
   end
@@ -14,11 +15,39 @@ defmodule Nectar.ModelExtension do
     end
   end
 
+  defmacro remove_from_schema([do: block]) do
+    to_remove = Macro.escape(block)
+    quote bind_quoted: [to_remove: to_remove] do
+      Module.put_attribute(__MODULE__, :remove_schema_changes, to_remove)
+    end
+  end
+
+
   defmacro __before_compile__(_env) do
     quote do
-      defmacro extensions do
-        @schema_changes
+
+      defmacro extendable_schema(source, [do: orig_schema]) do
+        final_schema = __process_schema_changes__(orig_schema)
+        quote do
+          schema unquote(source) do
+            unquote(final_schema)
+          end
+        end
       end
+
+      # rewrite the schema declaration
+      def __process_schema_changes__({:__block__, opts, declarations}) do
+        to_remove = @remove_schema_changes
+        to_add = @schema_changes
+        updated_declarations = Enum.filter(declarations, fn ({declaration_type, _declarataion_opt, declaration_params}) ->
+          # return false if found in to_remove list
+          !Enum.any?(to_remove, fn ({type, _opt, params}) ->
+            type == declaration_type && params == declaration_params
+          end)
+        end) ++ to_add  # append the fields to add at the end of declaration list
+        {:__block__, opts, updated_declarations}
+      end
+
     end
   end
 end
@@ -26,8 +55,11 @@ end
 defmodule Nectar.ExtendProduct do
   use Nectar.ModelExtension
 
-  add_to_schema do: (field :special, :boolean, virtual: true)
-  add_to_schema do: (field :type, :string, virtual: true)
+  add_to_schema      do: (field :special, :boolean, virtual: true)
+  add_to_schema      do: (field :type, :string, virtual: true)
+  remove_from_schema do: (field :slug, :string)
+  # must exactly match the declaration, no fuzzy matching supported
+  remove_from_schema do: (field :remove_me, :string, virtual: true)
 end
 
 defmodule Nectar.Product do
@@ -36,12 +68,15 @@ defmodule Nectar.Product do
   use Nectar.Web, :model
   use Arc.Ecto.Model
 
-  schema "products" do
+  extendable_schema "products" do
     field :name, :string
     field :description, :string
     field :available_on, Ecto.Date
     field :discontinue_on, Ecto.Date
+
+    # let's remove these two
     field :slug, :string
+    field :remove_me, :string, virtual: true
 
     has_one :master, Nectar.Variant, on_delete: :nilify_all # As this and below association same, how to handle on_delete
     has_many :variants, Nectar.Variant, on_delete: :nilify_all
@@ -52,7 +87,6 @@ defmodule Nectar.Product do
     has_many :product_categories, Nectar.ProductCategory
     has_many :categories, through: [:product_categories, :category]
 
-    extensions
     timestamps
   end