Add Single Table Inheritance to guides [ci skip]

guides/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   New section in Active Record Association Basics: Single Table Inheritance
+
+    *Andrey Nering*
+
 *   New section in Active Record Querying: Understanding The Method Chaining
 
     *Andrey Nering*

guides/source/association_basics.md

@@ -2243,3 +2243,67 @@ Extensions can refer to the internals of the association proxy using these three
 * `proxy_association.owner` returns the object that the association is a part of.
 * `proxy_association.reflection` returns the reflection object that describes the association.
 * `proxy_association.target` returns the associated object for `belongs_to` or `has_one`, or the collection of associated objects for `has_many` or `has_and_belongs_to_many`.
+
+Single Table Inheritance
+------------------------
+
+Sometimes, you may want to share fields and behavior between different models.
+Let's say we have Car, Motorcycle and Bicycle models. We will want to share
+the `color` and `price` fields and some methods for all of them, but having some
+specific behavior for each, and separated controllers too.
+
+Rails makes this quite easy. First, let's generate the base Vehicle model:
+
+```bash
+$ rails generate model vehicle type:string color:string price:decimal{10.2}
+```
+
+Did you note we are adding a "type" field? Since all models will be saved in a
+single database table, Rails will save in this column the name of the model that
+is being saved. In our example, this can be "Car", "Motorcycle" or "Bicycle."
+STI won't work without a "type" field in the table.
+
+Next, we will generate the three models that inherit from Vehicle. For this,
+we can use the `--parent=PARENT` option, which will generate a model that
+inherits from the specified parent and without equivalent migration (since the
+table already exists).
+
+For example, to generate the Car model:
+
+```bash
+$ rails generate model car --parent=Vehicle
+```
+
+The generated model will look like this:
+
+```ruby
+class Car < Vehicle
+end
+```
+
+This means that all behavior added to Vehicle is available for Car too, as
+associations, public methods, etc.
+
+Creating a car will save it in the `vehicles` table with "Car" as the `type` field:
+
+```ruby
+Car.create color: 'Red', price: 10000
+```
+
+will generate the following SQL:
+
+```sql
+INSERT INTO "vehicles" ("type", "color", "price") VALUES ("Car", "Red", 10000)
+```
+
+Querying car records will just search for vehicles that are cars:
+
+```ruby
+Car.all
+```
+
+will run a query like:
+
+```sql
+SELECT "vehicles".* FROM "vehicles" WHERE "vehicles"."type" IN ('Car')
+```