Mongoid batch inserts

In SQL land, all databases support batch inserts. Batch inserts are an effective and efficient mechanism to insert a lot of similar data. That is, instead of issuing x insert statements, you execute 1 insert with x records. This is much more efficient because the insert statement doesn’t need to be re-parsed x times, there is only 1 network trip as opposed to x, and in the case of transactions, there is only 1 transaction instead of x. When compared to x inserts, batch inserts are always faster.

As it turns out, MongoDB [1] supports batch inserts [2]! And just like in SQL land, Mongo’s batching feature is much faster at inserting a lot of data in one insert rather than x inserts.

For example, the Mongo Ruby driver [3]’s insert method takes a collection [4]; thus, you can insert an array of hashes quite efficiently. Even if you are using a ODM like Mongoid [5], you can still perform batch inserts as all you need to do is get a reference to the model object’s underlying collection and then issue an insert with an array of hashes matching the collection’s intended document structure.

For instance, to insert a collection of Tag models (each having 3 fields: name, system_tag, and account_id) in one fell swoop I can do the following:

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tags = ['a', 'bunch', 'of', 'tags'].collect { |tag| {name: tag, system_tag: true, account_id: id} }
Tag.collection.insert tags

In the code above, the insert takes a collection of hashes; what’s more, the insert is tied to the tags collection via the Tag.collection call.

Batch inserts are always faster if you have a lot of similar documents – in our case, we saw a tremendous performance increase when employing batching.