01 party model

class Party < ActiveRecord::Base
  has_many :projects, foreign_key: :owner_id
  has_and_belongs_to_many :projects
end

02 Project model

class Project < ActiveRecord::Base
  belongs_to :owner, class_name: "Party"
  has_and_belongs_to_many :contacts, class_name: "Party", foreign_key: "party_id"
end

03 Join table Project-party

class CreateJoinTableProjectParty < ActiveRecord::Migration
  def change
    create_join_table :projects, :parties do |t|
      t.index [:project_id, :party_id]
      t.index [:party_id, :project_id]
    end
  end
end

04 Error

> pr = Project.last
> pr.contacts
  Party Load (0.2ms)  SELECT "parties".* FROM "parties" INNER JOIN "parties_projects" ON "parties"."id" = "parties_projects"."party_id" WHERE "parties_projects"."party_id" = ?  [["party_id", 496074280]]
 => #<ActiveRecord::Associations::CollectionProxy []> 

> pr.contacts << Party.first
  Party Load (0.3ms)  SELECT  "parties".* FROM "parties"  ORDER BY "parties"."id" ASC LIMIT 1
   (0.2ms)  begin transaction
  SQL (1.3ms)  INSERT INTO "parties_projects" ("party_id") VALUES (?)  [["party_id", 397509679]]
   (0.2ms)  rollback transaction
ActiveRecord::StatementInvalid: SQLite3::ConstraintException: NOT NULL constraint failed: parties_projects.project_id: INSERT INTO "parties_projects" ("party_id") VALUES (?)

05 If I change Project.rb model to

class Project < ActiveRecord::Base
  belongs_to :owner, class_name: "Party"
  has_and_belongs_to_many :parties
end

... then everything works as expected