1 files changed, 43 insertions, 3 deletions
diff --git a/guides/source/association_basics.md b/guides/source/association_basics.md
index ec6017ff73..abac54d22d 100644
--- a/guides/source/association_basics.md
+++ b/guides/source/association_basics.md
@@ -146,6 +146,17 @@ class CreateSuppliers < ActiveRecord::Migration
 end
 ```
 
+Depending on the use case, you might also need to create a unique index and/or
+a foreign key constraint on the supplier column for the accounts table. In this
+case, the column definition might look like this:
+
+```ruby
+create_table :accounts do |t|
+  t.belongs_to :supplier, index: true, unique: true, foreign_key: true
+  # ...
+end
+```
+
 ### The `has_many` Association
 
 A `has_many` association indicates a one-to-many connection with another model. You'll often find this association on the "other side" of a `belongs_to` association. This association indicates that each instance of the model has zero or more instances of another model. For example, in an application containing customers and orders, the customer model could be declared like this:
@@ -829,6 +840,7 @@ The `belongs_to` association supports these options:
 * `:counter_cache`
 * `:dependent`
 * `:foreign_key`
+* `:primary_key`
 * `:inverse_of`
 * `:polymorphic`
 * `:touch`
@@ -875,18 +887,26 @@ end
 
 With this declaration, Rails will keep the cache value up to date, and then return that value in response to the `size` method.
 
-Although the `:counter_cache` option is specified on the model that includes the `belongs_to` declaration, the actual column must be added to the _associated_ model. In the case above, you would need to add a column named `orders_count` to the `Customer` model. You can override the default column name if you need to:
+Although the `:counter_cache` option is specified on the model that includes
+the `belongs_to` declaration, the actual column must be added to the
+_associated_ (`has_many`) model. In the case above, you would need to add a
+column named `orders_count` to the `Customer` model.
+
+You can override the default column name by specifying a custom column name in
+the `counter_cache` declaration instead of `true`. For example, to use
+`count_of_orders` instead of `orders_count`:
 
 ```ruby
 class Order < ActiveRecord::Base
   belongs_to :customer, counter_cache: :count_of_orders
 end
 class Customer < ActiveRecord::Base
-  has_many :orders, counter_cache: :count_of_orders
+  has_many :orders
 end
 ```
 
-NOTE: You only need to specify the :counter_cache option on the "has_many side" of the association when using a custom name for the counter cache.
+NOTE: You only need to specify the :counter_cache option on the `belongs_to`
+side of the association.
 
 Counter cache columns are added to the containing model's list of read-only attributes through `attr_readonly`.
 
@@ -913,6 +933,26 @@ end
 
 TIP: In any case, Rails will not create foreign key columns for you. You need to explicitly define them as part of your migrations.
 
+##### `:primary_key`
+
+By convention, Rails assumes that the `id` column is used to hold the primary key
+of its tables. The `:primary_key` option allows you to specify a different column.
+
+For example, given we have a `users` table with `guid` as the primary key. If we want a separate `todos` table to hold the foreign key `user_id` in the `guid` column, then we can use `primary_key` to achieve this like so:
+
+```ruby
+class User < ActiveRecord::Base
+  self.primary_key = 'guid' # primary key is guid and not id
+end
+
+class Todo < ActiveRecord::Base
+  belongs_to :user, primary_key: 'guid'
+end
+```
+
+When we execute `@user.todos.create` then the `@todo` record will have its
+`user_id` value as the `guid` value of `@user`.
+
 ##### `:inverse_of`
 
 The `:inverse_of` option specifies the name of the `has_many` or `has_one` association that is the inverse of this association. Does not work in combination with the `:polymorphic` options.