Multiple ActiveRecord Associations

Bear with me :-)

If I'm not mistaken:

• a user logs in and creates a trip
• each trip belongs to only one user
• each trip has multiple destinations

A first version could be:

class User
  has_many :trips
end

class Trip
  belongs_to :user
  has_many :destinations, inverse_of: :trip
end

class Destination
  belongs_to :trip
end

This way you can ask, given user Alice

• which trips did alice created?
• for each given trip, which destinations she added?

The problem with this one is that a destination belongs to a single trip, so if two people want to add "Bora Bora" to their trip, they can't, unless you duplicate the record.

To allow two trips to have Bora Bora you need to have a many-to-many relationship between them (a trip has multiple destinations, a destination can appear in multiple trips).

So our models would become:

class User
  has_many :trips
end

class Trip
  belongs_to :user

  has_many :trip_destinations
  has_many :destinations, through: :trip_destinations
end

class TripDestination # come up with a better name :D
  belongs_to :trip
  belongs_to :destination
end

class Destination
  has_many :trip_destinations
  has_many :trips, through: :trip_destinations
end

This way, you can know the following:

• which trips Alice has created
• which destinations belong to each trip
• which trips include each destination

In your database you will have a list of possible destinations and each user can add them to their trips.

Next, we need to let these users create their own destinations, instead of just using the pre-populated ones.

The only difference between a predefined destination and a user created is the notion of creator. Other than having a flag predefined or custom or something like that you need to attach the id of the creator to a destination.

Something like:

class User
  has_many :trips

  has_many :custom_destinations, class_name: "Destination", foreign_key: "creator_id"
end

class Trip
  belongs_to :user

  has_many :trip_destinations
  has_many :destinations, through: :trip_destinations
end

class TripDestination # come up with a better name :D
  belongs_to :trip
  belongs_to :destination
end

class Destination
  belongs_to :creator, class_name: "User", foreign_key: "creator_id"

  has_many :trip_destinations
  has_many :trips, through: :trip_destinations
end

The reason why I'm calling the foreign key creator_id and not user_id is for future proofing. Now you're mapping a relationship of creator/creation between a user and a destination, you might want to add another type of relationship between the two entities in the future, and add another key like moderator_id or whatever. They all point to the User table, they just point to separate concepts mapped to different foreign keys.

I haven't tested it but it should work. You need to create the various foreign keys in the migration:

• Destination.creator_id
• the join table's foreign keys between trip and destination, see many_to_many relationships in Rails

A tip: add the creation timestamp to the join table, so you'll know for free when the user has added the destination to their trip.

I hope I didn't forget something, let me know in case :-)

ps. my favorite gem to model permission (you're going to need something like it to describe the various actions a user can do, on destinations and comments) is Pundit