Don't hit database on uniqueness validation if the field has not chan…

…ged. Fixes #1576.

CHANGELOG.md

@@ -47,6 +47,9 @@ For instructions on upgrading to newer versions, visit
 
 ### Resolved Issues
 
+* \#1576 Don't hit database on uniqueness validation if the field getting
+  validated has not changed.
+
 * \#1568 Fallback to development environment with warning when no env configured.
 
 * \#1557 Internal strategy class no longer conflicts with models.

lib/mongoid/validations/uniqueness.rb

@@ -41,6 +41,7 @@ def setup(klass)
       #
       # @since 1.0.0
       def validate_each(document, attribute, value)
+        return unless document.send("#{attribute}_changed?")
         if document.embedded?
           return if skip_validation?(document)
           relation = document._parent.send(document.metadata.name)

spec/functional/mongoid/validations/uniqueness_spec.rb

@@ -65,8 +65,27 @@
                 Dictionary.create(:name => "Oxford")
               end
 
-              it "returns true" do
-                dictionary.should be_valid
+              context "when the field has changed" do
+
+                it "returns true" do
+                  dictionary.should be_valid
+                end
+              end
+
+              context "when the field has not changed" do
+
+                let(:from_db) do
+                  Dictionary.find(dictionary.id)
+                end
+
+                it "returns true" do
+                  from_db.should be_valid
+                end
+
+                it "does not touch the database" do
+                  Dictionary.expects(:where).never
+                  from_db.valid?
+                end
               end
             end
           end

spec/unit/mongoid/validations/uniqueness_spec.rb

@@ -11,7 +11,7 @@
     context "when the document is the root" do
 
       let(:dictionary) do
-        Dictionary.new(:year => 1999)
+        Dictionary.new(:year => 1999, :name => "")
       end
 
       let(:criteria) do
@@ -116,7 +116,7 @@
       end
 
       let(:definition) do
-        word.definitions.build
+        word.definitions.build(:description => "")
       end
 
       let(:criteria) do