Create associated record with foreign keys when another record is created in rails

Question

I have several models in rails that are associated in the following ways:

Models

class User < ApplicationRecord
  has_many :contracts
  has_many :properties, through: :contracts
  has_many :documents, through: :contracts

class Contract < ApplicationRecord
  belongs_to :seller_id, :class_name => 'Property'
  belongs_to :buyer_id, :class_name => 'User'
  has_many :documents
  has_one :property

class Property < ApplicationRecord
  has_many :contracts
  has_many :users

class Document < ApplicationRecord
  belongs_to :contract
  has_many :users, through: :contracts
  has_one :property, through: :contracts

The user who creates the property is saved under seller_id when it is created. Here's what I have so far:

Controllers

class DocumentsController < ApplicationController
    def create
      @contract = Contract.new()
      @document = current_user.documents.build(document_params)
      if @document.save
        flash[:notice] = "Successfully created document!"
        redirect_to documents_path
      else
        flash[:alert] = "Error creating document!"
        render :new
      end
    end

class ContractsController < ApplicationController
  def create
    @contract = Contract.new(contract_params)
    if @contract.save
      @contract.buyer_id << current_user
      @contract.seller_id << Property.find(params[:seller_id])
      @contract.property_id << Property.find(params[:id])
      flash[:notice] = "Successfully created contract!"
    else
      flash[:alert] = "Error creating contract!"
    end
  end

class PropertiesController < ApplicationController
   def create
    @property = current_user.properties.build(property_params)
    @property.user_id = current_user.id
    @property.seller_id = current_user.id
    if @property.save
      flash[:success] = "Successfully created property!"
      redirect_to property_path(@property)
    else
      flash[:alert] = "Error creating new property!"
      render :new
    end
  end

My Question

How would automatically create a contract record that associates the buyer_id to the current user, the seller_id to the seller of the property, and the property_id to the property when I create a new document in a form?

Please be thorough, I'm pretty new.

Thank you

Answer 1

class Contract < ApplicationRecord
  belongs_to :buyer, :class_name => 'User', foreign_key: buyer_id
  has_many :documents
  belongs_to :property, foreign_key: seller_id #belongs_to is used to define the relationship if foreign_key to the associated model is present. 

#Creating Contract
con = Contract.new(contract_params)
con.buyers << User.find(current_user.id) # links user to contract via. buyer_id
con.property << Property.find(params[:property_id]) # links property to contract via seller_id
con.save!

#Creating Document
doc = Document.new(document_params) 
doc.contracts << con # links contract to document
doc.save!