Rails N:M

Many to Many Associations

Objectives

• Implement many to many relationships through models in Rails

• Describe the model ordering opinion used by Rails

• Use the collection_check_boxes form helper to display a collection of associated items

Today we'll add rangers to the national park app using a many to many relationship.

Review: Relationships

Database relationships

What we need

• Models

  • Park

  • Ranger

  • ParksRangers (join table)

• Association

  • Park <-> ParksRangers <-> Ranger

  • Park has_and_belongs_to_many Rangers

  • Ranger has_and_belongs_to_many Parks

• Views

  • parks#edit - add/remove rangers checkboxes

  • parks#new - add/remove rangers checkboxes

  • rangers#show

    • list all parks with a specific ranger

Generating models

Review of Parks

rails g model park name description:text picture:text

Rangers

rails g model ranger name

ParksRangers

rails g model parks_rangers park:references ranger:references --force-plural

IMPORTANT

The join table with the two models must be plural and in alphabetical order if you want to follow the Rails convention. Also, --force-plural is needed so that the table will never be pluralized.

Note that if you want to name your join table something different, you can specify your own join model with through:

Setting up associations

When you do :references it automatically creates the belongs_to relations on the join table, but we need to manually add the has_and_belongs_to_many to the ranger and park models.

models/park.rb

has_and_belongs_to_many :rangers

models/ranger.rb

has_and_belongs_to_many :parks

ALSO IMPORTANT

When creating the M:M associations, the name of the model is pluralized when adding the has_and_belongs_to_many method. In ParksRangers, the associations will be singular and generated for you.

Adding rangers

# assume the following:
park = Park.first
ranger = Ranger.first

# adding a ranger
park.rangers << ranger

Removing rangers

# assume the following:
park = Park.first
ranger = Ranger.first

# clear all of the park's rangers (leaves the rangers in the table)
park.rangers.clear

# removes a specific ranger from a park (leaves the ranger in the table)
c.rangers.delete(ranger)

# removes a specific ranger from a park (and deletes the ranger)
c.rangers.first.destroy

Referencing and listing

Because Park and Ranger reference each other with has_and_belongs_to_many they can reference each other.

Basic Examples

#lists all rangers
Ranger.all

#lists all parks
Park.all

#gets first park in the database
Park.first

#lists all rangers of first park
Park.first.rangers

#lists all parks of first ranger
Ranger.first.parks

Advanced Examples / chaining

#All parks of the first ranger of the first park
Park.first.rangers.first.parks

parks#new and parks#edit

• Add checkboxes to the form

  <%= c.collection_check_boxes :ranger_ids, @rangers, :id, :name %>

• :ranger_ids refers to the model's rangers

• @rangers refers to all the rangers available (pass from the controller Ranger.all)

• :id refers to the value of the checkbox

• :name refers to the label of the checkbox

That's it! As far as assigning the rangers in the controller, we can modify the Park model to accept the ranger_ids array like so:

def park_params
  params.require(:park).permit(:name, :description, :ranger_ids => [])
end

In order for the rangers to be assigned automatically, we can add the accepts_nested_attributes_for method to the Park model. You'll also want to add inverse_of: to the park's ranger association, in order to run any validations that may be on the Ranger model.

class Park < ActiveRecord::Base
  has_and_belongs_to_many :rangers, inverse_of: :park
  accepts_nested_attributes_for :rangers
end

rangers#show

When showing a specific ranger, display the ranger name and the list of parks associated with it.