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copied!<p>I'm attempting to do something similar. I tried your suggestion of using the GROUP function, but I couldn't keep the embedded documents separate or was doing something incorrectly. </p> <p>I needed to pull/get a subset of embedded documents by ID. Here's how I did it using Map/Reduce:</p> <pre><code>db.parent.mapReduce( function(parent_id, child_ids){ if(this._id == parent_id) emit(this._id, {children: this.children, ids: child_ids}) }, function(key, values){ var toReturn = []; values[0].children.forEach(function(child){ if(values[0].ids.indexOf(product._id.toString()) != -1) toReturn.push(child); }); return {children: toReturn}; }, { mapparams: [ "4d93b112c68c993eae000001", //example parent id ["4d97963ec68c99528d000007", "4debbfd5c68c991bba000014"] //example embedded children ids ] } ).find() </code></pre> <p>I've abstracted my collection name to 'parent' and it's embedded documents to 'children'. I pass in two parameters: The parent document ID and an array of the embedded document IDs that I want to retrieve from the parent. Those parameters are passed in as the third parameter to the mapReduce function.</p> <p>In the map function I find the parent document in the collection (which I'm pretty sure uses the _id index) and emit its id and children to the reduce function.</p> <p>In the reduce function, I take the passed in document and loop through each of the children, collecting the ones with the desired ID. Looping through all the children is not ideal, but I don't know of another way to find by ID on an embedded document. </p> <p>I also assume in the reduce function that there is only one document emitted since I'm searching by ID. If you expect more than one parent_id to match, than you will have to loop through the <code>values</code> array in the reduce function.</p> <p>I hope this helps someone out there, as I googled everywhere with no results. Hopefully we'll see a built in feature soon from MongoDB, but until then I have to use this.</p>

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