has_and_belongs_to_many vs has_many through

My rule of thumb is, can I get by with a list of checkboxes here? If so, then it's a has-and-belongs-to-many (HABTM) association. If I need the checkbox to capture more about the relationship than simply yes/no it belongs, then use has_many :through. HABTM is as simple as using the _ids method with a simple_form collection_check_boxes. has_many :through often involves accepts_nested_attributes_for.

As far as I can remember, has_and_belongs_to_many gives you a simple lookup table which references your two models.

For example,

Stories can belong to many categories. Categories can have many stories.

Categories_Stories Table
story_id | category_id

has_many :through gives you a third model which can be used to store various other pieces of information which don't belong to either of the original models.

For example

Person can subscribe to many magazines. Magazines can have many subscribers.

Thus, we can have a subscription model in the middle, which gives us a similar table to the earlier example, but with additional properties.

Subscriptions Table
person_id | magazine_id | subscription_type | subscription_length | subscription_date 

And so on.

From http://guides.rubyonrails.org/association_basics.html#choosing-between-has-many-through-and-has-and-belongs-to-many

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.