Prinzhorn/documentation.md

## documentation.md

      
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    From http://docs.mongodb.org/manual/core/indexes/#sparse-indexes

Any document that is missing the field is not indexed.

--

You can combine the sparse index option with the unique indexes option so that mongod will reject documents that have duplicate values for a field, but that ignore documents that do not have the key.


## shell.js
> db.users.ensureIndex({ a: 1, b: 1 }, { sparse: true, unique: true });

> db.users.insert({a: 'a', b: 'b'});

> db.users.insert({a: 'a', b: 'b'});
E11000 duplicate key error index: uq.users.$a_1_b_1  dup key: { : "a", : "b" }
//As expected

> db.users.insert({a: 'a'});

> db.users.insert({a: 'a'});
E11000 duplicate key error index: uq.users.$a_1_b_1  dup key: { : "a", : null }
//DAFUG. It's sparse. Y U SET NULL?
	> db.users.ensureIndex({ a: 1, b: 1 }, { sparse: true, unique: true });

	> db.users.insert({a: 'a', b: 'b'});

	> db.users.insert({a: 'a', b: 'b'});
	E11000 duplicate key error index: uq.users.$a_1_b_1 dup key: { : "a", : "b" }
	//As expected

	> db.users.insert({a: 'a'});

	> db.users.insert({a: 'a'});
	E11000 duplicate key error index: uq.users.$a_1_b_1 dup key: { : "a", : null }
	//DAFUG. It's sparse. Y U SET NULL?