diff --git a/README.md b/README.md
index 563829a..ee3adf1 100644
--- a/README.md
+++ b/README.md
@@ -49,6 +49,36 @@ class AddUserDefinedFieldsToMyModel < ActiveRecord::Migration[7.0]
 end
 ```
 
+### Models
+Models that include the `UserDefinedFields::Fieldable` concern will be treated as the models that store the user defined data. The will be available in the dropdown list when configuring user defined fields.
+
+Applications that define user defined fields at the model level should call `resolve_defineable` class method with a lambda function that returns the `defineable` model.
+
+```ruby
+class MyModel < ApplicationRecord
+  include UserDefinedFields::Fieldable
+  
+  resolve_defineable -> (my_model) { my_model.parent }
+end
+```
+
+### Serializers
+To include the `user_defined_fields` array as a set of nested attributes, include the `UserDefinedFields::DefineableSerializer` in the serializer for whatever model the user defined fields belong to. This is only necessary for fields defined at the model level.
+
+```ruby
+class ParentModelSerializer < BaseSerializer
+  include UserDefinedFields::DefineableSerializer
+end
+```
+
+To include the `user_defined` hash as an attribute, include the `UserDefinedFields::FieldableSerializer` in the model serializer.
+
+```ruby
+class MyModelSerializer < BaseSerializer
+  include UserDefinedFields::FieldableSerializer
+end
+```
+
 ### Components
 
 User defined fields can be configured one of two ways: At the application level, or at the model level.
diff --git a/app/controllers/user_defined_fields/database_controller.rb b/app/controllers/user_defined_fields/database_controller.rb
index 55fa490..ac911ae 100644
--- a/app/controllers/user_defined_fields/database_controller.rb
+++ b/app/controllers/user_defined_fields/database_controller.rb
@@ -1,7 +1,7 @@
 module UserDefinedFields
   class DatabaseController < ApplicationController
     def data_types
-      data_types = %w(Boolean Date Number Select String Text)
+      data_types = %w(Boolean Date Number RichText Select String Text)
       render json: { data_types: data_types }, status: :ok
     end
 
diff --git a/app/models/concerns/user_defined_fields/fieldable.rb b/app/models/concerns/user_defined_fields/fieldable.rb
index 1fbf38e..3d24114 100644
--- a/app/models/concerns/user_defined_fields/fieldable.rb
+++ b/app/models/concerns/user_defined_fields/fieldable.rb
@@ -2,8 +2,52 @@ module UserDefinedFields
   module Fieldable
     extend ActiveSupport::Concern
 
+    class_methods do
+      def resolve_defineable(lambda = nil)
+        @resolve_defineable = lambda unless lambda.nil?
+        @resolve_defineable
+      end
+    end
+
     included do
+      # Resourceable parameters
       allow_params :user_defined
+
+      # Validations
+      validate :validate_user_defined_fields
+
+      private
+
+      def validate_user_defined_fields
+        defineable = self.class.resolve_defineable&.call(self)
+
+        query = UserDefinedField.all
+
+        if defineable
+          query = query.where(
+            defineable_id: defineable.id,
+            defineable_type: defineable.class.to_s
+          )
+        end
+
+        query.each do |field|
+          # Skip if this field is not required
+          next unless field.required?
+
+          # Parse the user defined field and extract the value
+          json = JSON.parse(self.user_defined || '{}')
+          value = json[field.column_name]
+
+          # Add an error if the value is "empty"
+          next unless value.nil? ||
+            (value.is_a?(String) && value.blank?) ||
+            (value.is_a?(Array) && value.length === 0) ||
+            (value.is_a?(Hash) && value.empty?)
+
+          message = I18n.t('errors.common.required', name: field.column_name)
+          self.errors.add(:user_defined, [field.column_name, message])
+        end
+      end
     end
   end
 end
diff --git a/app/serializers/concerns/user_defined_fields/defineable_serializer.rb b/app/serializers/concerns/user_defined_fields/defineable_serializer.rb
new file mode 100644
index 0000000..929e810
--- /dev/null
+++ b/app/serializers/concerns/user_defined_fields/defineable_serializer.rb
@@ -0,0 +1,10 @@
+module UserDefinedFields
+  module DefineableSerializer
+    extend ActiveSupport::Concern
+
+    included do
+      show_attributes user_defined_fields: UserDefinedFieldsSerializer
+    end
+
+  end
+end
diff --git a/app/serializers/concerns/user_defined_fields/fieldable_serializer.rb b/app/serializers/concerns/user_defined_fields/fieldable_serializer.rb
new file mode 100644
index 0000000..c93eefd
--- /dev/null
+++ b/app/serializers/concerns/user_defined_fields/fieldable_serializer.rb
@@ -0,0 +1,11 @@
+module UserDefinedFields
+  module FieldableSerializer
+    extend ActiveSupport::Concern
+
+    included do
+      index_attributes :user_defined
+      show_attributes :user_defined
+    end
+
+  end
+end
diff --git a/config/locales/en.yml b/config/locales/en.yml
new file mode 100644
index 0000000..21137b6
--- /dev/null
+++ b/config/locales/en.yml
@@ -0,0 +1,35 @@
+# Files in the config/locales directory are used for internationalization
+# and are automatically loaded by Rails. If you want to use locales other
+# than English, add the necessary files in this directory.
+#
+# To use the locales, use `I18n.t`:
+#
+#     I18n.t "hello"
+#
+# In views, this is aliased to just `t`:
+#
+#     <%= t("hello") %>
+#
+# To use a different locale, set it with `I18n.locale`:
+#
+#     I18n.locale = :es
+#
+# This would use the information in config/locales/es.yml.
+#
+# The following keys must be escaped otherwise they will not be retrieved by
+# the default I18n backend:
+#
+# true, false, on, off, yes, no
+#
+# Instead, surround them with single quotes.
+#
+# en:
+#   "true": "foo"
+#
+# To learn more, please read the Rails Internationalization guide
+# available at https://guides.rubyonrails.org/i18n.html.
+
+en:
+  errors:
+    common:
+      required: "%{name} is required"