Adding an additional model relationships using build() on Rails - ruby-on-rails

Question

Currently I have the following four models:
users <-- agency_memberships --> agencies
|
|
agency_roles
agency_memberships is a join table, and agency_roles is a small lookup table with the roles :
ID NAME
-----------
1 admin
2 editor
...
Before adding the AgencyRole model, when a user was created, if a param create_agency = true, then this was enough to create a new Agency (along the join table AgencyMembership).
# user_controller.rb
def create
#user = User.new(user_params)
#user.agencies.build(name: "Demo Agency") if params[:user][:create_agency]
if #user.save!
...
end
However, now I need to add a valid AgencyRole before saving.
Is it possible to do so with .build() or what is the Rails best practice to do so?
Right now, I'm creating all relationships manually before saving, which works but isn't as compact:
def create
#user = User.new(user_params)
if (params[:user][:create_agency])
agency_name = params[:user][:agency_name]
agency_role = AgencyRole.find_by(name: 'admin')
#agency_membership = AgencyMembership.new(agency_role: #agency_role)
#agency = Agency.new(name: agency_name)
#agency_membership.agency = #agency
#agency_membership.user = #user
#agency_membership.agency_role = agency_role
#user.agency_memberships << #agency_membership
end
if #user.save!
...
end
EdIT: My model relationships are as follows:
class AgencyMembership < ApplicationRecord
belongs_to :agency
belongs_to :user
belongs_to :agency_role
end
class AgencyRole < ApplicationRecord
validates :name, uniqueness: { case_sensitive: false }, presence: true
end
class Agency < ApplicationRecord
has_many :agency_memberships
has_many :projects
has_many :users, through: :agency_memberships
end
class User < ApplicationRecord
has_many :agency_memberships, inverse_of: :user
has_many :agencies, through: :agency_memberships
end

Answer 1

You can encapsulate and separate it from the controller, keep thin controller fat model, beside that you can use autosave in order to auto save association.
class AgencyMembership < ApplicationRecord
belongs_to :agency_role, autosave: true
end
class Agency < ApplicationRecord
has_one :agency_membership, autosave: true
end
module BuildAgency
def build_with_role(attributes = {}, &block)
agency_role_name = attributes.delete(:role)
agency = build(attributes, &block) # association: user - membership - agency
# add role
agency_role = AgencyRole.find_by(name: agency_role_name)
agency.agency_membership.agency_role = agency_role # autosave
agency
end
end
class User < ApplicationRecord
has_many :agencies, autosave: true, :extend => BuildAgency
def build_agency(attributes)
new_agency = agencies.build_with_role(attributes)
# ...
new_agency
end
end
# controller
def create
if (params[:user][:create_agency])
#user.build_agency(name: params[:user][:agency_name], role: params[:user][:agency_role])
end
if #user.save! # it'll save agencies since we set `autosave`
end