MONGOID-5103 Implement eq symbol operator

lib/mongoid/criteria/queryable/selectable.rb

@@ -254,6 +254,26 @@ def geo_spacial(criterion)
         end
         key :within_box, :override, "$geoWithin", "$box"
 
+        # Add the $eq criterion to the selector.
+        #
+        # @example Add the $eq criterion.
+        #   selectable.eq(age: 60)
+        #
+        # @example Execute an $eq in a where query.
+        #   selectable.where(:field.eq => 10)
+        #
+        # @param [ Hash ] criterion The field/value pairs to check.
+        #
+        # @return [ Selectable ] The cloned selectable.
+        def eq(criterion)
+          if criterion.nil?
+            raise Errors::CriteriaArgumentRequired, :eq
+          end
+
+          __override__(criterion, "$eq")
+        end
+        key :eq, :override, "$eq"
+
         # Add the $gt criterion to the selector.
         #
         # @example Add the $gt criterion.

spec/mongoid/criteria_spec.rb

@@ -1071,6 +1071,25 @@
     end
   end
 
+  describe "#eq" do
+
+    let!(:match) do
+      Band.create(member_count: 5)
+    end
+
+    let!(:non_match) do
+      Band.create(member_count: 1)
+    end
+
+    let(:criteria) do
+      Band.eq(member_count: 5)
+    end
+
+    it "returns the matching documents" do
+      expect(criteria).to eq([ match ])
+    end
+  end
+
   describe "#gt" do
 
     let!(:match) do