1.6 版新特性。
Changed in version 2.0: You may nest $or operations; however, these expressions are not as efficiently optimized as top-level.
Syntax: { $or: [ { <expression1> }, { <expression2> }, ... , { <expressionN> } ] }
The $or operator performs a logical OR operation on an array of two or more <expressions> and selects the documents that satisfy at least one of the <expressions>.
请看下面的查询：
db.inventory.find( { price:1.99, $or: [ { qty: { $lt: 20 } }, { sale: true } ] } )
此查询将会选择 inventory 集合中符合以下条件的所有文档：
Consider the following example which uses the $or operator to select fields from embedded documents:
db.inventory.update( { $or: [ { price:10.99 }, { "carrier.state": "NY"} ] }, { $set: { sale: true } } )
This update() operation will set the value of the sale field in the documents in the inventory collection where:
When using $or with <expressions> that are equality checks for the value of the same field, choose the $in operator over the $or operator.
请参考选择 inventory 集合中符合以下条件的所有文档的查询：
最有效的查询为：
db.inventory.find ( { $or: [ { price: 1.99 }, { sale: true } ], qty: { $in: [20, 50] } } )
Consider the following behaviors when using the $or operator:
When using indexes with $or queries, remember that each clause of an $or query will execute in parallel. These clauses can each use their own index. Consider the following query:
db.inventory.find ( { $or: [ { price: 1.99 }, { sale: true } ] } )
For this query, you would create one index on price ( db.inventory.ensureIndex( { price: 1 } ) ) and another index on sale ( db.inventory.ensureIndex( { sale: 1 } ) ) rather than a compound index.
Also, when using the $or operator with the sort() method in a query, the query will not use the indexes on the $or fields. Consider the following query which adds a sort() method to the above query:
db.inventory.find ( { $or: [ { price: 1.99 }, { sale: true } ] } ).sort({item:1})
这个修改后的查询将不使用关于 price 的索引，也不使用关于 sale 的索引。
You cannot use the $or with 2d geospatial queries.