how to do a has_many relation on two tables?

Question

I'm trying to do a has_many relation with a table on creation and also to add it in another table already created.

I have my User table already with no relations.

I want to create a Pro_user with a relation has_many User.

The thing is that one User can have multiple Pro_user and a Pro_user can also also have multiple User.

So both tables need a has_many right ?

So what I thought of was that

rails g model pro_user name:string email:string password_digest:string user:references

But this is not the right thing, it's doing a belongs_to from Pro_user to User

And also how should I do to do add the has_many on my existing table User ? Do I have to do a migration to recreate the table and adding the relation ?

Thanks for your help !

Answer 1

The recommended approach for a many to many association is the "has_many_through" approach. This allows you to add additional columns later on to the join table if you need more data. You'll need a join table that will at the least have two reference columns to your Users and ProUsers tables along with the standard id column. (Refer to: http://guides.rubyonrails.org/association_basics.html#the-has-many-through-association)

So your User and ProUser tables will not have any reference columns in them. Instead you'll make a third table called BoatsAndPros (call it whatever you like) and do:

    create_table :boats_and_pros do |t|
      t.belongs_to :user, index: true
      t.belongs_to :pro_user, index: true
      t.timestamps
    end

Then in the corresponding boats_and_pros.rb Model file you'll add:

belongs_to :user
belongs_to :pro_user

In your user.rb file you'll add:

 has_many :boats_and_pros
 has_many :pro_users, through: :boats_and_pros

In your pro_user.rb model file you'll add

has_many :boats_and_pros
has_many :users, through: :boats_and_pros

Two key takeaways are:

• The "oldschool" has_and_belongs_to_many approach is still fine however doesn't allow room to grow like the has_many_through approach here and you'll need to specifically name the table pro_users_users because Rails expects the two tables to be listed in lexical order.
• One-to-many relationships like what you ended up with on your original attempt keep the reference in one of the tables while many-to-many relationships require a third join table as shown above.