Add support for attribute default

JSON attributes can now use a default. Documentation has been added for the proper way to do it.

Closes #4.

lib/active_record/type/json.rb

@@ -29,10 +29,13 @@ def type_cast_for_database(value)
       private
 
       def cast_value(value)
-        if value.is_a?(::String)
+        case value
+        when ::String
           JSON.parse(value)
+        when Hash, Array
+          value
         else
-          raise "Unexpected JSON data type when loading from the database: #{value.class}"
+          raise "Unexpected JSON stored or default data type: #{value.class}"
         end
       end
     end

spec/json_on_rails/json_attributes_spec.rb

@@ -109,4 +109,48 @@
       end
     end
   end
+
+  # Smoke tests, to verify that the default works.
+  #
+  context "with Rails-defined defaults" do
+    # If a plain `{}` (or `[]`) is used as default, it will be shared on each model instantiation.
+    #
+    # This can be solved by using:
+    #
+    #     attribute :extras, ActiveRecord::Type::Json.new, default: -> { {} }
+    #
+    # but the following support would be required:
+    #
+    # - Type::Json: procs in #cast_value;
+    # - Proc: (Rails) marshalling, in case a dev wants to serialize the instance.
+    #
+    # which is not really worth.
+    #
+    context "on instantiation" do
+      it "should set the default" do
+        user_with_default = WithDefaultUser.new
+
+        expect do
+          user_with_default.extras["amiga500"] = "awesome"
+          user_with_default.save!
+        end.not_to raise_error
+      end
+    end
+
+    context "on create!" do
+      it "should set the default" do
+        user = WithDefaultUser.create!
+
+        expect { user.extras["amiga500"] = "awesome" }.not_to raise_error
+      end
+
+      it "should save the value when saving, even when not setting it" do
+        WithDefaultUser.create!
+
+        user = WithDefaultUser.find_by("extras LIKE ?", {}.to_json)
+
+        expect(user).not_to be(nil)
+      end
+    end
+  end
 end

spec/setup/db/schema.rb

@@ -1,7 +1,11 @@
 # frozen_string_literal: true
 
-ActiveRecord::Schema.define(version: 20180210000000) do
+ActiveRecord::Schema.define(version: 20180214000000) do
   create_table "users" do |t|
     t.json "extras"
   end
+
+  create_table "with_default_users" do |t|
+    t.json "extras"
+  end
 end

spec/setup/models/with_default_user.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class WithDefaultUser < ActiveRecord::Base
+  after_initialize do |instance|
+    instance.extras = {}
+  end
+end