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PODefining foreign key relationships for Rails' models
 

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  1. PODefining foreign key relationships for Rails' models
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    <p>I have a Comment class with a :foreign_key of post_id in the Post class.</p> <pre><code>class Comment &lt; ActiveRecord::Base belongs_to :post, :class_name =&gt; "Post", :foreign_key =&gt; "post_id", :counter_cache =&gt; true belongs_to :author, :class_name =&gt; "User", :foreign_key =&gt; "author_id" end </code></pre> <p><strong>But my CreateComments migration does not define a database-level foreign key:</strong></p> <pre><code>class CreateComments &lt; ActiveRecord::Migration def self.up create_table :comments do |t| t.column "post_id", :integer, :default =&gt; 0, :null =&gt; false t.column "author", :string, :default =&gt; "", :limit =&gt; 25, :null =&gt; false t.column "author_email", :string, :default =&gt; "", :limit =&gt; 50, :null =&gt; false t.column "content", :text, :null =&gt; false t.column "status", :string, :default =&gt; "", :limit =&gt; 25, :null =&gt; false t.timestamps end end def self.down drop_table :comments end end </code></pre> <p>Instead post_id is a simple Integer column. </p> <p><strong>So, it seems that this foreign key relationship exists only in the mind of Rails, not at the database level.</strong> </p> <p>Is this correct?</p> <p><strong>Also, is it necessary for the corresponding Post model to also declare its reciprocal foreign key relationship with Comments using the :foreign_key attribute or could that be omitted?</strong></p> <pre><code>class Post &lt; ActiveRecord::Base set_table_name("blog_posts") belongs_to :author, :class_name =&gt; "User", :foreign_key =&gt; 'author_id' has_many :comments, :class_name =&gt; "Comment", :foreign_key =&gt; 'post_id', :order =&gt; "created_at desc", :dependent =&gt; :destroy has_many :categorizations has_many :categories, :through =&gt; :categorizations named_scope :recent, :order =&gt; "created_at desc", :limit =&gt; 5 end </code></pre>
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    1. COI share your surprise that Rails doesn't use SQL foreign keys. It makes it difficult to use non-rails tools on DBs.
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      1. USGreg
    2. CORails follows convention that "ALL business logic should be defined in the application"... so it uses DB only as a "dumb" storage. No foreign keys, no stored procedures, no constraints (supported in postgres for example). EDIT: Just found this answer which says the same - http://stackoverflow.com/questions/8334602/need-to-create-a-foreign-key-when-creating-a-table-on-rails
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      1. USDalibor Filus
 

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