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POHow to deal with Many-to-Many relations in MongoDB when Embedding is not the answer?
 

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  1. POHow to deal with Many-to-Many relations in MongoDB when Embedding is not the answer?
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    <p>Here's the deal. Let's suppose we have the following data schema in MongoDB:</p> <ul> <li><code>items</code>: a collection with large documents that hold some data (it's absolutely irrelevant what it actually is).</li> <li><code>item_groups</code>: a collection with documents that contain a list of <code>items._id</code> called <code>item_groups.items</code> plus some extra data.</li> </ul> <p>So, these two are tied together with a Many-to-Many relationship. But there's one tricky thing: for a certain reason I cannot store items within item groups, so -- just as the title says -- embedding is not the answer.</p> <p>The query I'm really worried about is intended to find some particular groups that contain some particular items (i.e. I've got a set of criteria for each collection). In fact it also has to say how much items within each found group fitted the criteria (no items means group is not found).</p> <p>The only viable solution I came up with this far is to use a Map/Reduce approach with a dummy reduce function:</p> <pre><code>function map () { // imagine that item_criteria came from the scope. // it's a mongodb query object. item_criteria._id = {$in: this.items}; var group_size = db.items.count(item_criteria); // this group holds no relevant items, skip it if (group_size == 0) return; var key = this._id.str; var value = {size: group_size, ...}; emit(key, value); } function reduce (key, values) { // since the map function emits each group just once, // values will always be a list with length=1 return values[0]; } db.runCommand({ mapreduce: item_groups, map: map, reduce: reduce, query: item_groups_criteria, scope: {item_criteria: item_criteria}, }); </code></pre> <p>The problem line is:</p> <pre><code>item_criteria._id = {$in: this.items}; </code></pre> <p>What if this.items.length == 5000 or even more? My RDBMS background cries out loud:</p> <pre><code>SELECT ... FROM ... WHERE whatever_id IN (over 9000 comma-separated IDs) </code></pre> <p><strong>is definitely not a good way to go</strong>.</p> <p>Thank you sooo much for your time, guys!</p> <p>I hope the best answer will be something like "you're stupid, stop thinking in RDBMS style, use <strong>$its_a_kind_of_magicSphere</strong> from the latest release of MongoDB" :)</p>
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    1. USEwan Doe
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      1. POHow to deal with Many-to-Many relations in MongoDB when Embedding is not the answer?
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    1. COWhen talking about "embedding" are you talking about DBReferences --> http://www.mongodb.org/display/DOCS/Database+References#DatabaseReferences-Javascript%28mongoshell%29?
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      1. POHow to deal with Many-to-Many relations in MongoDB when Embedding is not the answer?
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    2. COI'm talking about the actual embedding i.e. storing one document within another. e.g. comment = {user: 'DrColossos', text: 'what are you talking about?'}; question = {x: 13, y: 42, comments: [comment]}.
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      1. POHow to deal with Many-to-Many relations in MongoDB when Embedding is not the answer?
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    3. COIf you create a Many-to-Many relation with an intermediate table and query that? wont that solve your problem?
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