Validation context should default to proper context instead of none:

- When no context provided to Document#valid? use the following
  defaults: :create if new, :update if persisted.

- Slight refactoring of the include annoyance in the validations module.

- Mongoid now includes ActiveModel::Validations::Callbacks

- Initialize callback runs only :after.

- Fixes #556.

lib/mongoid/callbacks.rb

@@ -4,24 +4,10 @@ module Callbacks
     extend ActiveSupport::Concern
     included do
       extend ActiveModel::Callbacks
+      include ActiveModel::Validations::Callbacks
 
-      define_model_callbacks \
-        :create,
-        :destroy,
-        :initialize,
-        :save,
-        :update,
-        :validation
-    end
-
-    # Determine if the document is valid.
-    #
-    # @example Is the document valid?
-    #   person.valid?
-    #
-    # @return [ true, false ] True if valid, false if not.
-    def valid?(*)
-      _run_validation_callbacks { super }
+      define_model_callbacks :initialize, :only => :after
+      define_model_callbacks :create, :destroy, :save, :update
     end
   end
 end

lib/mongoid/validations.rb

@@ -8,46 +8,57 @@ module Mongoid #:nodoc:
   # provide: validates_associated and validates_uniqueness_of.
   module Validations
     extend ActiveSupport::Concern
+    include ActiveModel::Validations
 
-    included do
-      include ActiveModel::Validations
+    attr_accessor :validated
 
-      attr_accessor :validated
-
-      # Overrides the default ActiveModel behaviour since we need to handle
-      # validations of relations slightly different than just calling the
-      # getter.
-      #
-      # @todo Durran: Why does moving the ActiveModel::Validations include
-      #   statement outside of the block bomb the test suite. This feels dirty.
-      #
-      # @example Read the value.
-      #   person.read_attribute_for_validation(:addresses)
-      #
-      # @param [ Symbol ] attr The name of the field or relation.
-      #
-      # @return [ Object ] The value of the field or the relation.
-      #
-      # @since 2.0.0.rc.1
-      def read_attribute_for_validation(attr)
-        if relations[attr.to_s]
-          send(attr, false, :eager => true)
-        else
-          send(attr)
-        end
+    # Overrides the default ActiveModel behaviour since we need to handle
+    # validations of relations slightly different than just calling the
+    # getter.
+    #
+    # @example Read the value.
+    #   person.read_attribute_for_validation(:addresses)
+    #
+    # @param [ Symbol ] attr The name of the field or relation.
+    #
+    # @return [ Object ] The value of the field or the relation.
+    #
+    # @since 2.0.0.rc.1
+    def read_attribute_for_validation(attr)
+      if relations[attr.to_s]
+        send(attr, false, :eager => true)
+      else
+        send(attr)
       end
+    end
 
-      # Used to prevent infinite loops in associated validations.
-      #
-      # @example Is the document validated?
-      #   document.validated?
-      #
-      # @return [ true, false ] Has the document already been validated?
-      #
-      # @since 2.0.0.rc.2
-      def validated?
-        !!@validated
-      end
+    # Determine if the document is valid.
+    #
+    # @example Is the document valid?
+    #   person.valid?
+    #
+    # @example Is the document valid in a context?
+    #   person.valid?(:create)
+    #
+    # @param [ Symbol ] context The optional validation context.
+    #
+    # @return [ true, false ] True if valid, false if not.
+    #
+    # @since 2.0.0.rc.6
+    def valid?(context = nil)
+      super context ? context : (new? ? :create : :update)
+    end
+
+    # Used to prevent infinite loops in associated validations.
+    #
+    # @example Is the document validated?
+    #   document.validated?
+    #
+    # @return [ true, false ] Has the document already been validated?
+    #
+    # @since 2.0.0.rc.2
+    def validated?
+      !!@validated
     end
 
     module ClassMethods #:nodoc:

spec/integration/mongoid/validations_spec.rb

@@ -0,0 +1,41 @@
+require "spec_helper"
+
+describe Mongoid::Validations do
+
+  let(:account) do
+    Account.new(:name => "Testing a really long name.")
+  end
+
+  describe "#valid?" do
+
+    context "when provided a context" do
+
+      it "uses the provided context" do
+        account.should be_valid(:update)
+      end
+    end
+
+    context "when not provided a context" do
+
+      context "when the document is new" do
+
+        it "defaults the context to :create" do
+          account.should_not be_valid
+        end
+      end
+
+      context "when the document is persisted" do
+
+        before do
+          account.name = "Testing"
+          account.save
+          account.name = "Testing a really long name."
+        end
+
+        it "defaults the context to :update" do
+          account.should be_valid
+        end
+      end
+    end
+  end
+end

spec/models/account.rb

@@ -12,4 +12,5 @@ class Account
   attr_accessible :nickname, :name
 
   validates_presence_of :name
+  validates_length_of :name, :maximum => 10, :on => :create
 end