5 files changed, 54 insertions, 31 deletions
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index acd3d395c5..f7718394af 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,11 @@
+*   Treat blank UUID values as `nil`.
+
+    Example:
+
+        Sample.new(uuid_field: '') #=> <Sample id: nil, uuid_field: nil>
+
+    *Dmitry Lavrov*
+
 *   Enable support for materialized views on PostgreSQL >= 9.3.
 
     *Dave Lee*
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/oid.rb b/activerecord/lib/active_record/connection_adapters/postgresql/oid.rb
index 57bdc3bb19..9e898015a6 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/oid.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/oid.rb
@@ -330,6 +330,13 @@ This is not reliable and will be removed in the future.
           end
         end
 
+        class Uuid < Type
+          def type; :uuid end
+          def type_cast(value)
+            value.presence
+          end
+        end
+
         class TypeMap
           def initialize
             @mapping = {}
@@ -418,10 +425,10 @@ This is not reliable and will be removed in the future.
         register_type 'json', OID::Json.new
         register_type 'cidr', OID::Cidr.new
         register_type 'inet', OID::Inet.new
+        register_type 'uuid', OID::Uuid.new
         register_type 'xml', SpecializedString.new(:xml)
         register_type 'tsvector', SpecializedString.new(:tsvector)
         register_type 'macaddr', SpecializedString.new(:macaddr)
-        register_type 'uuid', SpecializedString.new(:uuid)
         register_type 'citext', SpecializedString.new(:citext)
         register_type 'ltree', SpecializedString.new(:ltree)
 
diff --git a/activerecord/test/cases/adapters/postgresql/uuid_test.rb b/activerecord/test/cases/adapters/postgresql/uuid_test.rb
index f79a7a598b..9e03ea6bee 100644
--- a/activerecord/test/cases/adapters/postgresql/uuid_test.rb
+++ b/activerecord/test/cases/adapters/postgresql/uuid_test.rb
@@ -50,6 +50,11 @@ class PostgresqlUUIDTest < ActiveRecord::TestCase
     assert_not column.array
   end
 
+  def test_treat_blank_uuid_as_nil
+    UUIDType.create! guid: ''
+    assert_equal(nil, UUIDType.last.guid)
+  end
+
   def test_uuid_formats
     ["A0EEBC99-9C0B-4EF8-BB6D-6BB9BD380A11",
      "{a0eebc99-9c0b-4ef8-bb6d-6bb9bd380a11}",
diff --git a/guides/CHANGELOG.md b/guides/CHANGELOG.md
index a7c215c295..246e1d3b96 100644
--- a/guides/CHANGELOG.md
+++ b/guides/CHANGELOG.md
@@ -1 +1,5 @@
+*   Switched the order of `Applying a default scope` and `Merging of scopes` subsections so default scopes are introduced first.
+
+    *Alex Riabov*
+
 Please check [4-1-stable](https://github.com/rails/rails/blob/4-1-stable/guides/CHANGELOG.md) for previous changes.
diff --git a/guides/source/active_record_querying.md b/guides/source/active_record_querying.md
index 0a332d7dd9..2a76df156c 100644
--- a/guides/source/active_record_querying.md
+++ b/guides/source/active_record_querying.md
@@ -1231,6 +1231,35 @@ Using a class method is the preferred way to accept arguments for scopes. These
 category.posts.created_before(time)
 ```
 
+### Applying a default scope
+
+If we wish for a scope to be applied across all queries to the model we can use the
+`default_scope` method within the model itself.
+
+```ruby
+class Client < ActiveRecord::Base
+  default_scope { where("removed_at IS NULL") }
+end
+```
+
+When queries are executed on this model, the SQL query will now look something like
+this:
+
+```sql
+SELECT * FROM clients WHERE removed_at IS NULL
+```
+
+If you need to do more complex things with a default scope, you can alternatively
+define it as a class method:
+
+```ruby
+class Client < ActiveRecord::Base
+  def self.default_scope
+    # Should return an ActiveRecord::Relation.
+  end
+end
+```
+
 ### Merging of scopes
 
 Just like `where` clauses scopes are merged using `AND` conditions.
@@ -1284,36 +1313,6 @@ User.where(state: 'inactive')
 As you can see above the `default_scope` is being merged in both
 `scope` and `where` conditions.
 
-
-### Applying a default scope
-
-If we wish for a scope to be applied across all queries to the model we can use the
-`default_scope` method within the model itself.
-
-```ruby
-class Client < ActiveRecord::Base
-  default_scope { where("removed_at IS NULL") }
-end
-```
-
-When queries are executed on this model, the SQL query will now look something like
-this:
-
-```sql
-SELECT * FROM clients WHERE removed_at IS NULL
-```
-
-If you need to do more complex things with a default scope, you can alternatively
-define it as a class method:
-
-```ruby
-class Client < ActiveRecord::Base
-  def self.default_scope
-    # Should return an ActiveRecord::Relation.
-  end
-end
-```
-
 ### Removing All Scoping
 
 If we wish to remove scoping for any reason we can use the `unscoped` method. This is