mongodb Mongodb查询选择具有给定键的记录
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Mongodb Query To select records having a given key
提问by Sagar Varpe
The records in my database are
我数据库中的记录是
{"_id":"1","fn":"sagar","ln":"Varpe"}
{"_id":"1","fn":"sag","score":"10"}
{"_id":"1","ln":"ln1","score":"10"}
{"_id":"1","ln":"ln2"}
I need to design a MongoDB query to find all records that have a given key.
我需要设计一个 MongoDB 查询来查找具有给定键的所有记录。
For example, if I pass lnas a parameter to the query it shuold return all records in which lnis a key. The results would be
例如，如果我ln作为参数传递给查询，它应该返回所有ln作为键的记录。结果将是
{"_id":"1","fn":"sagar","ln":"Varpe"}
{"_id":"1","ln":"ln1","score":"10"}
{"_id":"1","ln":"ln2"}
回答by Justin Jenkins
回答by HADEEL
I had the same problem and
我有同样的问题
db.coll.find({"mykey":{'$exists': 1}})
worked for me
对我来说有效
回答by helpdoc
db.collection.find({ ln: { $exists: true} });
The $size operator matches any array with the number of elements specified by the argument. For example:
$size 运算符匹配具有参数指定的元素数量的任何数组。例如：
db.collection.find({ ln: { $exists: true, $size: 0 } });
$size does not accept ranges of values. To select documents based on fields with different numbers of elements, create a counter field that you increment when you add elements to a field.
$size 不接受值范围。要根据具有不同元素数量的字段选择文档，请创建一个计数器字段，在向字段添加元素时增加该字段。
