Proposal Creation

project.rb

class Project < ActiveRecord::Base
  # Associations
  has_and_belongs_to_many :users
  
  has_many :proposals, dependent: :destroy
end

proposal.rb

class Proposal < ActiveRecord::Base
  # Associations
  belongs_to :project
  belongs_to :user
end

proposals_controller.rb

class ProposalsController < ApplicationController
  before_filter :find_project, only: [:new]
  
  def create
    @proposal = Proposal.create(params[:proposal])
  end
  
  # I am making the assumption here that you want a form to make a new proposal, this is that page  
  def new
    @proposal = current_user.proposals.new(:project => @project)
  end
  
  private
  
  def find_project
    # This ensures the user has access to the project
    @project = current_user.projects.find(params[:project_id])
  end
end

routes.rb

resources :projects do
  # This will create a route /projects/[:project_id]/proposals/new
  # Note, [:project_id] may actually be [:id]. If it is, just substitute that in the proposals controller
  
  resources :proposals, only: [:create, :new]
end

user.rb

# I'm changing around your schema a bit I think, but I saw you wanted to allow multiple users to create proposals for projects in the future

class User < ActiveRecord::Base
  # Associations
  has_and_belongs_to_many :projects # Can also do a has_many :projects_users, :has_many :projects, through: :projects_users
  
  has_many :proposals, dependent: :destroy
end