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copied!<p>Say you have two models: <code>User</code> and <code>Group</code>.</p> <p>If you wanted to have users belong to groups, then you could do something like this:</p> <pre><code>class Group < ActiveRecord::Base has_many :users end class User < ActiveRecord::Base belongs_to :group end </code></pre> <p>What if you wanted to track additional metadata around the association? For example, when the user joined the group, or perhaps what the user's role is in the group?</p> <p>This is where you make the association a first class object:</p> <pre><code>class GroupMembership < ActiveRecord::Base belongs_to :user belongs_to :group # has attributes for date_joined and role end </code></pre> <p>This introduces a new table, and eliminates the <code>group_id</code> column from the user's table.</p> <p>The problem with this code is that you'd have to update every where else you use the user class and change it:</p> <pre><code>user.groups.first.name # becomes user.group_memberships.first.group.name </code></pre> <p>This type of code sucks, and it makes introducing changes like this painful.</p> <p><strong><code>has_many :through</code></strong> gives you the best of both worlds:</p> <pre><code>class User < ActiveRecord::Base has_many :groups, :through => :group_memberships # Edit :needs to be plural same as the has_many relationship has_many :group_memberships end </code></pre> <p>Now you can treat it like a normal <code>has_many</code>, but get the benefit of the association model when you need it.</p> <p>Note that you can also do this with <code>has_one</code>.</p> <p><strong>Edit: Making it easy to add a user to a group</strong></p> <pre><code>def add_group(group, role = "member") self.group_associations.build(:group => group, :role => role) end </code></pre>

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