1) class CreateTodos < ActiveRecord::Migration
 def change
 create_table :todos do |t|
 t.string :text
 t.boolean :isCompleted
 t.references :project, foreign_key: true
 t.timestamps
 end
 end
 end

  2) class CreateProjects < ActiveRecord::Migration[5.2]
  def change
  create_table :projects do |t|
  t.string :title
  t.timestamps
  end
  end
  end

Rails.application.routes.draw do
root 'projects#index'
resources :projects
resources :todos
end

<div class="form-box">
 <div class="form-box-wrapper">
  <%= form_with scope: :todo, url: todos_path, local: true do |form| %>
  <%= form.text_field :text, placeholder: 'Название задачи' %>
  <%= select(:post, :project_id, Project.all.collect{|p| [ p.title, p.id ] }, {include_blank: 'None'}) %>
  <br>
  <%= form.submit %>
  <a href="/" id="candel_button" class="cansel">ОТМЕНА</a>
  <% end %>
  </div>
  </div>

class Todo < ApplicationRecord
    belongs_to :project, optional: true
end

class Project < ApplicationRecord
    has_many :todos
end

def create
  @todo = Todo.new(todo_params)
  if @todo.save
    redirect_to @todo
  else
    render :edit
  end
end

private

def todo_params
    params[:todo].permit(:text, :project_id)
end

<div class="form-box">
  <div class="form-box-wrapper">
    <%= form_with model: @todo, local: true do |form| %>
    <%= form.text_field :text, placeholder: 'Название задачи' %>
    <%= form.select(:project_id, Project.all.collect{|p| [ p.title, p.id ] }, {include_blank: 'None'}) %>
    <br>
    <%= form.submit %>
    <a href="/" id="candel_button" class="cansel">ОТМЕНА</a>
  <% end %>
</div>