本文介绍了MongoDB按字段查找对象的数组(联接条件和不相关的子查询)的处理方法,对大家解决问题具有一定的参考价值,需要的朋友们下面随着小编来一起学习吧!
问题描述
我不了解使用连接条件和不相关子查询.
处理
集合:
{
_id: 'p1',
steps: [
{
_id: 'ps1',
step: 's1',
time: 10
},
{
_id: 'ps2',
step: 's2',
time: 15
}
]
}
steps
集合(对于具有 _id:s1
的文档):
steps
collection (for the document with _id: s1
):
{
_id: 's1',
name: 'step 1'
}
工作汇总(标准一种,没有加入条件和不相关的子查询):
Working aggregation (standard one, without join conditions and uncorrelated sub-queries):
processes.aggregate([
{
// match stage or whatever prior stage
},
{
$lookup: {
from: 'steps',
localField: 'steps.step',
foreignField: '_id',
as: 'steps'
}
}
])
输出:
{
_id: 'p1',
steps: [
{
_id: 's1',
name: 'step 1'
},
{
_id: 's2',
name: 'step 2'
}
]
}
不起作用的聚合:
processes.aggregate([
{
// match stage or whatever prior stage
},
{
$lookup: {
from: 'steps',
let: { stepId: '$steps.step' }, // I think the problem is here
pipeline: [
{
$match: {
$expr: { $eq: ['$_id', '$$stepId'] },
},
},
{
// Additional stages here
}
],
as: 'steps',
},
}
])
输出:
{
_id: 'p1',
steps: []
}
推荐答案
steps.step
在这种情况下计算为字符串数组 ["s1","s2"] 代码>.常规的
$ lookup
支持这种比较,并且在后台进行 $ in
.
steps.step
evaluates to an array of strings in this case ["s1", "s2"]
. The regular $lookup
supports such comparison and does $in
behind the scenes.
在第二个示例中,您使用的是 $ expr
,因此您需要使用表达式语言,因此必须使用 $ in 运算符:
In your second example you're using $expr
so you need to use expression language hence you have to use $in operator:
$expr: { $in: ['$_id', '$$stepId'] }
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