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POMongoDB: How to merge two collections/databases together into one?
 

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  1. POMongoDB: How to merge two collections/databases together into one?
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    <p>First please note this question is different from the following two</p> <p><a href="https://stackoverflow.com/questions/5681851/mongodb-combine-data-from-multiple-collections-in-to-one-how">MongoDB: Combine data from multiple collections into one..how?</a></p> <p><a href="https://stackoverflow.com/questions/9696940/merging-two-collections-in-mongodb">Merging two collections in MongoDB</a></p> <p><a href="https://stackoverflow.com/questions/4067197/mongodb-and-joins">MongoDB and &quot;joins&quot;</a></p> <p>I will explain as follows:</p> <p>I have two databases named: <code>DB_A</code> and <code>DB_B</code> in my mongodb. </p> <p>Each database has one collection with same name called <code>store</code>. </p> <p>Both collections have lots lots of docs that have exactly the same structure {key:"key1", value:"value1"}, etc.</p> <p>Actually, I was supposed to only create <code>DB_A</code> and insert all docs into <code>DB_A</code>. But later when I did my second round of inserting, I made a mistake by typing the wrong name as the database name.</p> <p>So now, each database has size of 32GB, I wish to merge two databases.</p> <p>One problem/constraint is that the free space available now is only 15GB, so I can't just <code>copy</code> all things from <code>DB_B</code> to <code>DB_A</code>. </p> <p>I am wondering whether I can do <code>move</code>? or what is the simplest / best / <strong>fast</strong> way to merge the two simple databases? I prefer the most efficient way as simply reinserting 32GB into <code>DB_A</code> will take quite a time.</p>
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    1. COTake first document from DB_B. Insert it into DB_A if not already there. Delete it from DB_B. Repeat until done.
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    2. CO@SergioTulentsev If I just delete DB_B and reinsert everything (of DB_B) freshly into DB_A using my code, will this way be slower than the one you suggested?
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    3. COIf all your data is in DB_B, then you can drop DB_A and just rename DB_B.
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