Has May Through Polymorphic

has_many_tp.rb

class User
  has_many :group_memberships
  has_many :groups, through: :group_memberships
  has_many :project_memberships
  has_many :projects, through: :project_memberships
  # etc...
end

class Project
  has_many :project_memberships
  has_many :users, through: :project_memberships
end

class Group
  has_many :group_memberships
  has_many :users, through: :group_memberships
end