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    <p>This looks like a simple m:n relationship with a two-part key. Model it relationally with a joint table. To give you the idea, assuming there can only be one <code>(post, attachment)</code> pair per c-ordinate location, the SQL phrasing would be:</p> <pre><code>CREATE TABLE post ( post_id serial primary key, blah_other_data text ); CREATE TABLE attachment ( attachment_id serial primary key, blah_other_data text ); CREATE TABLE post_attachment ( post_id integer not null REFERENCES post(post_id), attachment_id integer not null REFERENCES attachment(attachment_id), coordinate_x integer not null, coordinate_y integer not null, PRIMARY KEY (coordinate_x, coordinate_y) ); </code></pre> <p>I'm sure you can translate that back into Rails models and queries.</p> <p>If there can be multiple <code>(post,attachment)</code> pairs per co-ordinate location then you must adjust the <code>PRIMARY KEY</code> of <code>post_attachments</code> to permit that by adding the co-ordinate columns to the key.</p> <p>Demo data setup:</p> <pre><code>INSERT INTO post (blah_other_data) VALUES ('Post1'),('Post2'),('Post3'); INSERT INTO attachment(blah_other_data) VALUES ('Attachment1'),('Attachment2'),('Attachment3'); INSERT INTO post_attachment (post_id, attachment_id, coordinate_x, coordinate_y) SELECT post_id, attachment_id, x, y FROM (VALUES ('Post1', 100,200, 'Attachment1'), ('Post1', 400,400, 'Attachment2'), ('Post1', 200,500, 'Attachment3'), ('Post2', 150,310, 'Attachment1'), ('Post3', 50,710, 'Attachment1'), ('Post1', 430,430, 'Attachment2') ) rows(post_data,x,y,attachment_data) INNER JOIN post ON (post.blah_other_data = rows.post_data) INNER JOIN attachment ON (attachment.blah_other_data = rows.attachment_data); </code></pre> <p>Access the data with joins:</p> <pre><code>SELECT p.post_id, p.blah_other_data AS post_data, a.attachment_id, a.blah_other_data AS attachment_data, c.coordinate_x, c.coordinate_y FROM post_attachment c INNER JOIN post p ON (c.post_id = p.post_id) INNER JOIN attachment a ON (c.attachment_id = a.attachment_id); post_id | post_data | attachment_id | attachment_data | coordinate_x | coordinate_y ---------+-----------+---------------+-----------------+--------------+-------------- 3 | Post3 | 1 | Attachment1 | 50 | 710 2 | Post2 | 1 | Attachment1 | 150 | 310 1 | Post1 | 1 | Attachment1 | 100 | 200 1 | Post1 | 2 | Attachment2 | 400 | 400 1 | Post1 | 2 | Attachment2 | 430 | 430 1 | Post1 | 3 | Attachment3 | 200 | 500 (6 rows) </code></pre>
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    1. POWhat is the way to store array of object_id and its coordinates in database?
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    1. USCraig Ringer
    1. USCraig Ringer
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    1. COThank you, I'll try!
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    2. COIs [this](http://compositekeys.rubyforge.org) the proper way to implement complex primary keys?
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    3. CO@sadfuzzy I'm not into Rails and ActiveRecord particularly, so I can't really help with that part. I'm stunned that it doesn't support composite keys out of the box. If you need to you can change the data model to add a synthetic primary key, and make the co-ordinates `UNIQUE (coordinate_x, coordinate_y)` instead.
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