build expression not(nil) as IS NOT NULL

lib/arel/engines/sql/core_extensions/nil_class.rb

@@ -5,6 +5,10 @@ module Arel
         'IS'
       end
 
+      def not_predicate_sql
+        'IS NOT'
+      end
+
       NilClass.send(:include, self)
     end
   end

lib/arel/engines/sql/core_extensions/object.rb

@@ -9,6 +9,10 @@ module Arel
         '='
       end
 
+      def not_predicate_sql
+        '!='
+      end
+
       Object.send(:include, self)
     end
   end

lib/arel/engines/sql/predicates.rb

@@ -27,7 +27,9 @@ module Arel
     end
 
     class Not < Binary
-      def predicate_sql; '!=' end
+      def predicate_sql
+        operand2.not_predicate_sql
+      end
     end
 
     class GreaterThanOrEqualTo < Binary

lib/arel/engines/sql/primitives.rb

@@ -34,6 +34,10 @@ module Arel
       value.equality_predicate_sql
     end
 
+    def not_predicate_sql
+      value.not_predicate_sql
+    end
+
     def to_sql(formatter = Sql::WhereCondition.new(relation))
       formatter.value value
     end

spec/engines/sql/unit/predicates/not_spec.rb

@@ -0,0 +1,75 @@
+require 'spec_helper'
+
+module Arel
+  module Predicates
+    describe Equality do
+      before do
+        @relation1 = Arel::Table.new(:users)
+        @relation2 = Arel::Table.new(:photos)
+        @attribute1 = @relation1[:id]
+        @attribute2 = @relation2[:user_id]
+      end
+
+      describe '#to_sql' do
+        describe 'when relating to a non-nil value' do
+          it "manufactures a not predicate" do
+            sql = Not.new(@attribute1, @attribute2).to_sql
+
+            adapter_is :mysql do
+              sql.should be_like(%Q{`users`.`id` != `photos`.`user_id`})
+            end
+
+            adapter_is :oracle do
+              sql.should be_like(%Q{"USERS"."ID" != "PHOTOS"."USER_ID"})
+            end
+
+            adapter_is_not :mysql, :oracle do
+              sql.should be_like(%Q{"users"."id" != "photos"."user_id"})
+            end
+          end
+        end
+
+        describe 'when relation to a nil value' do
+          before do
+            @nil = nil
+          end
+
+          it "manufactures an is null predicate" do
+            sql = Not.new(@attribute1, @nil).to_sql
+
+            adapter_is :mysql do
+              sql.should be_like(%Q{`users`.`id` IS NOT NULL})
+            end
+
+            adapter_is :oracle do
+              sql.should be_like(%Q{"USERS"."ID" IS NOT NULL})
+            end
+
+            adapter_is_not :mysql, :oracle do
+              sql.should be_like(%Q{"users"."id" IS NOT NULL})
+            end
+          end
+        end
+
+        describe "when relating to a nil Value" do
+          it "manufactures an IS NULL predicate" do
+            value = nil.bind(@relation1)
+            sql = Not.new(@attribute1, value).to_sql
+
+            adapter_is :mysql do
+              sql.should be_like(%Q{`users`.`id` IS NOT NULL})
+            end
+
+            adapter_is :oracle do
+              sql.should be_like(%Q{"USERS"."ID" IS NOT NULL})
+            end
+
+            adapter_is_not :mysql, :oracle do
+              sql.should be_like(%Q{"users"."id" IS NOT NULL})
+            end
+          end
+        end
+      end
+    end
+  end
+end