Convert comparison to operation when setting a query operation

This fixes the problem where building a query using intersection
starting with a query that matches everything ends up with an exception.

In example:

  users = User.all
  users &= User.all(:name => 'foo')
  users &= User.all(:email => /bar')

lib/dm-core/query.rb

@@ -1356,7 +1356,18 @@ def set_operation(operation, other)
     #
     # @api private
     def other_conditions(other, operation)
-      query_conditions(self).send(operation, query_conditions(other))
+      self_conditions = query_conditions(self)
+
+      unless self_conditions.kind_of?(Conditions::Operation)
+        operation_slug = case operation
+                         when :intersection, :difference then :and
+                         when :union                     then :or
+                         end
+
+        self_conditions = Conditions::Operation.new(operation_slug, self_conditions)
+      end
+
+      self_conditions.send(operation, query_conditions(other))
     end
 
     # Extract conditions from a Query

spec/semipublic/query_spec.rb

@@ -1960,7 +1960,15 @@ class ::Other
           end
         end
 
-        subject { @query.send(method, @other) }
+        subject do
+          result = @query.send(method, @other)
+
+          if @another
+            result = result.send(method, @another)
+          end
+
+          result
+        end
 
         describe 'with equivalent query' do
           before { @other = @query.dup }
@@ -1995,8 +2003,9 @@ class ::Other
 
         describe 'with self matching everything' do
           before do
-            @query = DataMapper::Query.new(@repository, @model)
-            @other = DataMapper::Query.new(@repository, @model, :name => 'Dan Kubb')
+            @query   = DataMapper::Query.new(@repository, @model)
+            @other   = DataMapper::Query.new(@repository, @model, :name => 'Dan Kubb')
+            @another = DataMapper::Query.new(@repository, @model, :citizenship => 'US')
           end
 
           it { should be_kind_of(DataMapper::Query) }
@@ -2005,10 +2014,10 @@ class ::Other
 
           it { should_not equal(@other) }
 
+          it { should_not equal(@another) }
+
           it 'should factor out the operation matching everything' do
-            pending 'TODO: compress Query#conditions for proper comparison' do
-              should == DataMapper::Query.new(@repository, @model, :name => 'Dan Kubb')
-            end
+            should == DataMapper::Query.new(@repository, @model, :name => 'Dan Kubb', :citizenship => 'US')
           end
         end