class Contract
  include Mongoid::Document

  has_and_belongs_to_many :signatures

  field :item, type: String
end

class Signature
  include Mongoid::Document

  has_and_belongs_to_many :contracts

  field :name, type: String
  field :year, type: Integer
end

c=Contract.create!
s=Signature.create!(contracts:[c])
s.contracts.first
# => #<Contract _id: 5c5dda8e026d7c1730969846, signature_ids: [BSON::ObjectId('5c5dda9a026d7c1730969847')], item: nil>

class HabtmmContract
  include Mongoid::Document

  has_and_belongs_to_many :signatures, class_name: 'HabtmmSignature'

  field :item, type: String
end

class HabtmmSignature
  include Mongoid::Document

  has_and_belongs_to_many :contracts, class_name: 'HabtmmContract'

  field :name, type: String
  field :year, type: Integer
end

contract = HabtmmContract.create!(item: 'foo')
s=HabtmmSignature.create!(contracts: [contract], name: 'Dave', year: 2019)
s.contracts.first
# => #<HabtmmContract _id: 5c5dda46026d7c1730969844, signature_ids: nil, item: "foo">

      contract = HabtmmContract.create!(item: 'foo')
      contract.signatures << HabtmmSignature.create!(contracts: [contract], name: 'Dave', year: 2019)
      contract.save!