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    <p>Here's a complete answer to this issue, in case people visiting this question are new to Ruby on Rails and having a hard time putting everything together (as I was when I first looked into this).</p> <p>Some parts of the solution take place in your Migrations and some in your Models:</p> <h2>Migrations</h2> <pre><code>class CreatePrivateMessages &lt; ActiveRecord::Migration def change create_table :private_messages do |t| t.references :sender t.references :recipient end # Rails 5+ only: add foreign keys add_foreign_key :private_messages, :users, column: :sender_id, primary_key: :id add_foreign_key :private_messages, :users, column: :recipient_id, primary_key: :id end end </code></pre> <p>Here you are specifying that there are two columns in this table that will be referred to as :sender and :recipient and which hold references to another table. Rails will actually create columns called 'sender_id' and 'recipient_id' for you. In our case they will each reference rows in the Users table, but we specify that in the models, not in the migrations.</p> <h2>Models</h2> <pre><code>class PrivateMessage &lt; ActiveRecord::Base belongs_to :sender, :class_name =&gt; 'User' belongs_to :recipient, :class_name =&gt; 'User' end </code></pre> <p>Here you are creating a property on the PrivateMessage model named :sender, then specifying that this property is related to the User class. Rails, seeing the "belongs_to :sender", will look for a column in your database called "sender_id", which we defined above, and use that to store the foreign key. Then you're doing the exact same thing for the recipient.</p> <p>This will allow you to access your Sender and Recipient, both instances of the User model, through an instance of the PrivateMessage model, like this: </p> <pre><code>@private_message.sender.name @private_message.recipient.email </code></pre> <p>Here is your User Model:</p> <pre><code>class User &lt; ActiveRecord::Base has_many :sent_private_messages, :class_name =&gt; 'PrivateMessage', :foreign_key =&gt; 'sender_id' has_many :received_private_messages, :class_name =&gt; 'PrivateMessage', :foreign_key =&gt; 'recipient_id' end </code></pre> <p>Here you are creating a property on the User Model named :sent_private_messages, specifying that this property is related to the PrivateMessage Model, and that the foreign key on the PrivateMessage model which relates it to this property is called 'sender_id'. Then you are doing the same thing for received private messages.</p> <p>This allows you to get all of a users sent or received private messages by doing something like this:</p> <pre><code>@user.sent_private_messages @user.received_private_messages </code></pre> <p>Doing either of these will return an array of instances of the PrivateMessage model.</p> <p>....</p>
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    1. PORuby on rails - Reference the same model twice?
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    1. USEric Andrew Lewis
    1. USRichard Jones
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    1. COThe `class_name` keys for the `PrivateMessage` model should be symbols I can't edit this myself because edits must be at least 6 characters.
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      1. USTim Fletcher
    2. CO@TimFletcher The examples on this page all use strings, not symbols: http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-belongs_to. Maybe the `to_s` message is sent to a passed Symbol (just guessing here)?
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