[英]MongoDB Aggregate $lookup on array of object with additional field
I have two different collections (example below) methods & items .我有两种不同的 collections(下面的示例)方法和项目。 As for now, I'm using pre 3.6 vanilla aggregation query for
$lookup
:至于现在,我正在使用 3.6 之前的香草聚合查询
$lookup
:
MongoPlayground Example MongoPlayground 示例
{
$lookup: {
from: "items",
localField: "reagents._id",
foreignField: "_id",
as: "reagent_items"
}
}
The problem is that if I am using it, I miss quantity
field (from methods.reagents
embedded) during $lookup
stage from original collection.问题是,如果我使用它,我会在原始集合的
$lookup
阶段错过quantity
字段(来自嵌入的methods.reagents
)。 For now, I return quantity
right after lookup
but as I heard, Mongo introduced from 3.6 a new syntax for lookup
queries, so the question is:目前,我在
lookup
后立即返回quantity
,但正如我听说的那样,Mongo 从 3.6 引入了一种用于lookup
查询的新语法,所以问题是:
Can it solve my problem for receiving the following results:
它能否解决我收到以下结果的问题:
{
"_id": 1,
"name": "Test",
"reagent_items": [ // <= the exact schema of what I need after lookup
{
"_id": 1,
"name": "ItemOne",
"other": "field",
"quantity": 2 //quantity field from original {array of objects} after lookup
},
{
"_id": 2,
"name": "ItemTwo",
"other": "field",
"quantity": 4 //quantity field from original {array of objects} after lookup
}
],
"reagents": [ //original reagents field here just for example, we could remove it
{
"_id": 1,
"quantity": 2
},
{
"_id": 2,
"quantity": 4
}
]
}
methods方法
{
"_id": 1,
"name": "Test",
"reagents": [
{
_id: 1,
quantity: 2
},
{
_id: 2,
quantity: 4
}
]
}
items项目
{
"_id": 1,
"name": "ItemOne",
"other": "field"
},
{
"_id": 2,
"name": "ItemTwo",
"other": "field"
}
Use $map along with $arrayElemAt to find corresponding reagent
for each reagent_items
and the apply $mergeObjects to get one object:使用$map和$arrayElemAt为每个
reagent_items
找到对应的reagent
,并应用$mergeObjects获得一个 object:
db.methods.aggregate([
{
$lookup: {
from: "items",
localField: "reagents._id",
foreignField: "_id",
as: "reagent_items"
}
},
{
$project: {
_id:1,
name: 1,
reagents: 1,
reagent_items: {
$map: {
input: "$reagent_items",
as: "ri",
in: {
$mergeObjects: [
"$$ri",
{
$arrayElemAt: [ { $filter: { input: "$reagents", cond: { $eq: [ "$$this._id", "$$ri._id" ] } } }, 0 ]
}
]
}
}
}
}
}
])
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