Rails: How to cache a field of a has_many relationship in an array?

The has_many :through association is also useful for setting up “shortcuts” through nested has_many associations. For example, if a document has many sections, and a section has many paragraphs, you may sometimes want to get a simple collection of all paragraphs in the document. A has_one :through association sets up a one-to-one connection with another model.

This association indicates that the declaring model can be matched with one instance of another model by proceeding through a third model. A has_and_belongs_to_many association creates a direct many-to-many connection with another model, with no intervening model. If you want to set up a one-to-one relationship between two models, you’ll need to add belongs_to to one, and has_one to the other.

How do you know which is which? The distinction is in where you place the foreign key (it goes on the table for the class declaring the belongs_to association), but you should give some thought to the actual meaning of the data as well. The has_one relationship says that one of something is yours – that is, that something points back to you.

For example, it makes more sense to say that a supplier owns an account than that an account owns a supplier. Rails offers two different ways to declare a many-to-many relationship between models. The second way to declare a many-to-many relationship is to use has_many :through.

The simplest rule of thumb is that you should set up a has_many :through relationship if you need to work with the relationship model as an independent entity. If you don’t need to do anything with the relationship model, it may be simpler to set up a has_and_belongs_to_many relationship (though you’ll need to remember to create the joining table in the database). You should use has_many :through if you need validations, callbacks, or extra attributes on the join model.

A slightly more advanced twist on associations is the polymorphic association. With polymorphic associations, a model can belong to more than one other model, on a single association.