Mongo使用

在用mongo进行查询时，$exists表示是否document是否包含这个field，即使field的value为null也算是包含。

$existsSyntax: { field: { $exists: <boolean> } }When <boolean> is true, $exists matches the documents that contain the field, including documents where the field value is null. If <boolean> is false, the query returns only the documents that do not contain the field.ExamplesExists and Not Equal ToConsider the following example:db.inventory.find( { qty: { $exists: true, $nin: [ 5, 15 ] } } )This query will select all documents in the inventory collection where the qty field exists and its value does not equal 5 or 15.Null ValuesGiven a collection named records with the following documents:{ a: 5, b: 5, c: null }{ a: 3, b: null, c: 8 }{ a: null, b: 3, c: 9 }{ a: 1, b: 2, c: 3 }{ a: 2, c: 5 }{ a: 3, b: 2 }{ a: 4 }{ b: 2, c: 4 }{ b: 2 }{ c: 6 }Consider the output of the following queries:Query: db.records.find( { a: { $exists: true } } )Result: { a: 5, b: 5, c: null }{ a: 3, b: null, c: 8 }{ a: null, b: 3, c: 9 }{ a: 1, b: 2, c: 3 }{ a: 2, c: 5 }{ a: 3, b: 2 }{ a: 4 }Query: db.records.find( { b: { $exists: false } } )Result: { a: 2, c: 5 }{ a: 4 }{ c: 6 }Query: db.records.find( { c: { $exists: false } } )Result: { a: 3, b: 2 }{ a: 4 }{ b: 2 }

而要查看document中哪些field的值为null，则需要用$in 来查询。详情可以参考http://docs.mongodb.org/manual/reference/operator/query/in/ 页面