This is a classic User -> Membership -> Group example with some twist:
User
is a member of a single Group at any given time. But I want to keep the
history which Group the User was a member of.
I am looking for a proper way of doing it.
This is how I think models should look like: #user.rb
Class User < ActiveRecord::Base
has_many :memberships
has_many :groups, through: :memberships
#membership.rb
Class Membership < ActiveRecord::Base
belongs_to :user
belongs_to :group
#group.rb
Class Group < ActiveRecord::Base
has_many :memberships
has_many :users, through: :memberships
Obviously we need date_from attribute in memberships to keep a track on
when the user joined the group.
Creating/Updating is working out of the box. It’s querying data I have
trouble with.
Now it is easy to get the last group the user is a member of: #user.rb
…
def latest_group
memberships.order(:date_from).last.group
end
Similarly you can get latest user for a group.
But how to get all users currently belonging to group? Furthermore how
to
get all the users belonging to group at given date?
An option I’ve used in the past is to add a datetime field such as
“dropped_at” that defaults to null to your memberships table.
From there, you could add a scope to the memberships model like:
class Membership < ActiveRecord::Base
scope :active, -> { where(dropped_at: nil) }
end
This concept used to be used when you wanted to “soft delete” a record
from
your table,
but have an option to “undo” the delete. Here, in your instance - you
can
borrow the idea
to easily find those memberships for a given User that are “active” as
well
as those users
that are currently a part of a group. The standard created_at field
that
is on the memberships
table can be used to indicate when a user joined a group.