Group operations for MongoDB

Insert test data first:

For (VAR I = 1; I <20; I ++ ){

VaR num = I % 6;

DB. Test. insert ({_ ID: I, name: "User _" + I, age: num });

}

1. Common grouping query (http://www.my400800.cn)

DB. Test. Group ({key: {age: true}, initial: {num: 0}, $ reduce: function (Doc, Prev ){

Prev. Num ++

}});

DB. runcommand ({group:

{

NS: "test ",

Key: {age: true },

Initial: {num: 0 },

$ Reduce: function (Doc, Prev ){

Prev. Num ++}

}

});

2. filter and then group

DB. Test. Group ({key: {age: true}, initial: {num: 0}, $ reduce: function (Doc, Prev ){

Prev. Num ++

},

Condition: {age :{$ GT: 2 }}

});

DB. runcommand ({group:

{

NS: "test ",

Key: {age: true },

Initial: {num: 0 },

$ Reduce: function (Doc, Prev ){

Prev. Num ++ },

Condition: {age :{$ GT: 2 }}

}

});

Common $ where queries:

DB. Test. Find ({$ where: function (){

Return this. age> 2;

}

});

Group join $ where Query

DB. Test. Group ({key: {age: true}, initial: {num: 0}, $ reduce: function (Doc, Prev ){

Prev. Num ++

},

Condition: {$ where: function (){

Return this. age> 2;

}

}

});

3. grouping with function return values

// Note that the $ keyf specified function must return an object.

DB. test. group ({$ keyf: function (DOC) {return {age: Doc. age };}, initial: {num: 0}, $ reduce: function (Doc, Prev ){

Prev. Num ++

}

});

DB. runcommand ({group:

{

NS: "test ",

$ Keyf: function (DOC) {return {age: Doc. Age };},

Initial: {num: 0 },

$ Reduce: function (Doc, Prev ){

Prev. Num ++}

}

});

4. Use the Terminator

DB. test. group ({$ keyf: function (DOC) {return {age: Doc. age };}, initial: {num: 0}, $ reduce: function (Doc, Prev ){

Prev. Num ++

},

Finalize: function (DOC) {Doc. Count = Doc. Num; Delete Doc. Num ;}

});

DB. runcommand ({group:

{

NS: "test ",

$ Keyf: function (DOC) {return {age: Doc. Age };},

Initial: {num: 0 },

$ Reduce: function (Doc, Prev ){

Prev. Num ++ },

Finalize: function (DOC) {Doc. Count = Doc. Num; Delete Doc. Num ;}

}

});