Ruby on Rails - Models and Relationship Table

Question

I am a noob starting out with ruby on rails and trying to understand models. Pretty familiar with databases and wanted to understand when to generate models for a relationship?

For example - I have users table and gadgets table. Every user can have multiple gadgets. I want to store this relationship in a relationship table with user_id and gadget_id. I have created the two tables by using the rails generate model User and rails generate model Gadget cmd.

Question - Will I now create another model called users_gadgets ? Is the naming convention correct? I have seen lots of documentation on how to create associations (has_many, belongs_to ) but don't understand when I need to create model?

Do I have to create models every time I want to store a relationship in a database and then run my migrations?

Answer 1

A relation model only makes sense for a N-to-N relationship as you described it. When you choose to use has_and_belongs_to_many, you don't need to create a join model, but you still have to create the join table via a migration. The naming convention is then gadgets_users, because 'g' is before 'u' in the alphabet.

If you chose has_many :through, you could store additional information within the join table via a GadgetUser join model. For example if you wanted to record that a gadget belonged to a user for a given time, then this time information logically belongs to the "connection" between gadget and user so you should keep it in the join table.

Answer 2

you can do it one of two ways

1) has_and_belongs_to_many in this case you just need a table (no model) gadgets_users with user_id gadget_id, rails likes the table names to be plural in alpha order

2) has_many :through, this is not a 'normal' has many, in this case you would need a model (can be named whatever you like) i prefer this one over the other because it allows you to add additional attributes to the relationship if needed

see the guide, it does a way better job explaining than I can http://guides.rubyonrails.org/association_basics.html#choosing-between-has_many-through-and-has_and_belongs_to_many

also - railscasts - http://railscasts.com/episodes/47-two-many-to-many

Do I have to create models every time I want to store a relationship in a database and then run my migrations? Yes, most of the time