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POForeign key contraints in many-to-many relationships
 

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    <h2>Context</h2> <p>We're building a blog for an intro. to databases course project.</p> <p>In our blog, we want to be able to set <code>Labels</code> on <code>Posts</code>. The <code>Labels</code> can't exist by themselves, they only do so if they are related to a <code>Posts</code>. This way, <code>Labels</code> that are not used by any <code>Posts</code> shouldn't stay in the database.</p> <p>More than one <code>Label</code> can belong to a single <code>Post</code>, and more than a single <code>Post</code> can use a <code>Label</code>.</p> <p>We are using both SQLite3 (locally/testing) and PostgreSQL (deployment).</p> <h2>Implementation</h2> <p>Here is the SQL (SQLite3 flavor) that we use to create those two tables, along with the relationship table:</p> <h3>Posts</h3> <pre><code>CREATE TABLE IF NOT EXISTS Posts( id INTEGER PRIMARY KEY AUTOINCREMENT, authorId INTEGER, title VARCHAR(255), content TEXT, imageURL VARCHAR(255), date DATETIME, FOREIGN KEY (authorId) REFERENCES Authors(id) ON DELETE SET NULL ) </code></pre> <h3>Labels</h3> <pre><code>CREATE TABLE IF NOT EXISTS Labels( id INTEGER PRIMARY KEY AUTOINCREMENT, name VARCHAR(255) UNIQUE, -- This is not working: FOREIGN KEY (id) REFERENCES LabelPosts(labelId) ON DELETE CASCADE ) </code></pre> <p><strong>LabelPosts</strong> (relation between <code>Post</code> [1..*] -- * <code>Label</code>)</p> <pre><code>CREATE TABLE IF NOT EXISTS LabelPosts( postId INTEGER, labelId INTEGER, PRIMARY KEY (postId, labelId), FOREIGN KEY (postId) REFERENCES Posts(id) ON DELETE CASCADE ) </code></pre> <h2>Problem</h2> <ul> <li><p>Using SQLite3, <code>Labels</code> are not deleted from the database when I remove all references to it from the <code>LabelPosts</code> table. I think for the reason given by Postgres, despite SQLite accepting the table without warning.</p></li> <li><p>PostgreSQL complains that <code>labelId</code> is not unique within <code>LabelPosts</code>, which is true and also required, since it's many-to-many:</p></li> </ul> <blockquote> <p>pq: S:"ERROR" R:"transformFkeyCheckAttrs" L:"6511" C:"42830" F:"tablecmds.c"<br> M:"there is no unique constraint matching given keys for referenced table \"labelposts\""</p> </blockquote> <p>So I understand that I'm doing my constraint wrong. However I don't know how to do it properly.</p>
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    1. USErwin Brandstetter
    1. USAntoineG
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    1. COAre you allowed to use triggers?
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    2. COYes I am, but I'm not too familiar with them.
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    3. COThis does not look like a very standard approach of doing things. I can see what you're trying to achieve, however the foreign key constraint on table `Labels` does not fit. You should do `CONSTRAINT fk_LabelPosts_labelId FOREIGN KEY(labelId) REFERENCES Labels(labelId)` on `LabelPosts` instead (in my mind). Some other minor observations: 1. It is often advised to name your entities in singular form, ie. `Label`, `Post` and `LabelPost` in this case. 2. I would recommend naming all constraints - this will make it easier to DROP unneccessary constraints later.
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