asdf.rb

class User < Sequel::Model
  one_to_many :memberships
  many_to_many :teams, :join_table => :memberships
end

class Membership < Sequel::Model
  many_to_one :user
  many_to_one :team
end

class Team < Sequel::Model
  one_to_many :memberships
  many_to_many :users, :join_table => :memberships

  def membership_id
    # ???
  end

  def hash_with_membership_id
    hash = values
    hash[:membership_id] = membership_id
    hash
  end
end

User.create(:id => 1)
Team.create(:id => 1)
Membership.create(:id => 1, :user_id => 1, :team_id => 1)

User[1].teams.collect { |t| t.hash_with_membership_id } # [{:id => 1, :membership_id => 1}]