按一个字段进行汇总,然后选择具有另一个字段最大值的文档作为集合 [英] Aggregating by a field and selecting the document with the max value of another field as a collection
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问题描述
使用聚合框架,什么是获取每个分组中字段最大值的文档的最佳方法,因此使用下面的集合,我希望具有为每个具有最新日期的group_id返回一个文档的功能.第二个清单显示了预期的结果.
Using the aggregate framework, what is the best way to get documents with a maximum value of a field per grouping so using the collection below I would like to have functionality to return one document for each group_id having the latest date. The second listing shows the desired result.
group_id date
1 11/1/12
1 11/2/12
1 11/3/12
2 11/1/12
3 11/2/12
3 11/3/12
期望结果
group_id date
1 11/3/12
2 11/1/12
3 11/3/12
推荐答案
您可以使用 $max
聚合框架中的分组功能,以查找每个group_id
的最新文档.您将需要其他查询才能根据分组的条件检索完整的文档.
You can use the $max
grouping function in the Aggregation Framework to find the latest document for each group_id
. You will need additional queries to retrieve the full documents based on the grouped criteria.
var results = new Array();
db.groups.aggregate(
// Find documents with latest date for each group_id
{ $group: {
_id: '$group_id',
date: { $max: '$date' },
}},
// Rename _id to group_id, so can use as find criteria
{ $project: {
_id: 0,
group_id:'$_id',
date: '$date'
}}
).result.forEach(function(match) {
// Find matching documents per group and push onto results array
results.push(db.groups.findOne(match));
});
示例结果:
{
"_id" : ObjectId("5096cfb8c24a6fd1a8b68551"),
"group_id" : 1,
"date" : ISODate("2012-11-03T00:00:00Z"),
"foo" : "bar"
}
{
"_id" : ObjectId("5096cfccc24a6fd1a8b68552"),
"group_id" : 2,
"date" : ISODate("2012-11-01T00:00:00Z"),
"foo" : "baz"
}
{
"_id" : ObjectId("5096cfddc24a6fd1a8b68553"),
"group_id" : 3,
"date" : ISODate("2012-11-03T00:00:00Z"),
"foo" : "bat"
}
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