Is there a way to limit the number of records in certain collection

As开发者_Go百科suming I insert the following records (e.g. foo1, foo2, foo3, foo4, .... foo10) I would like the collection to retain only 5 records at any point in time (e.g. it could be foo1, ... foo5 OR foo2, ... foo6 or foo6, ... foo10)

How should I achieve this?

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Sounds like you're looking for a capped collection:

Capped collections are fixed sized collections...
[...]
Once the space is fully utilized, newly added objects will replace the oldest objects in the collection.

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You can achieve this using a command similar to

db.createCollection("collectionName",{capped:true,size:10000,max:5})

where 10000 is the size in bytes and 5 is the maximum number of documents you would restrict in the collection.

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By using
db.createCollection("collectionName",{capped:true,size:10000,max:1000})

command, limits can be put on the no. of records in collection

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Capped Collections sound great but it limits remove operation. In a capped Collection you can only insert new elements and old elements will be removed automatically when certain size is reached.

However if you want to insert and delete any document in any order and limit the maximum number of document in a collection, mongoDB does not offer a direct solution.

When I encounter this problem I use an extra count variable in another collection. This collection has a validation rule that avoids count variables to become negative. Count variable should always be non negative integer number.

"count": { "$gte": 0 }

The algorithm was simple. Decrement the count by one. If it succeed insert the document. If it fails it means there is no space left.

Vice versa for deletion.

Also you can use transactions to prevent failures(Count is decremented but service is failed just before insertion operation).