英文:
Golang mongodb aggregation
问题
我有一个用户集合。用户有一个 int64 的 "id"，还有 "avatar"、"name" 和其他用户 id 的数组。
我想要实现的是查询单个用户，但是不仅仅获取他的朋友 id 数组，还要获取他的朋友数组，包含他们的姓名和头像。在 golang 中如何实现这个目标？我找到了一些类似我需要的东西 - "lookup" 函数，但是我不知道如何正确使用它。
英文:
I have a collection of users. User have an int64 "id", and lets say "avatar", "name" and an array of other users ids.
What I want to achieve is to query SINGLE user, but instead of getting an array with his friends ids I want to get an array of his friends, containing their names and avatars. How to achieve it in golang? I have found some kind of what I need - "lookup" function, but I cannot understand how to use it right.
答案1
得分: 8
你不能直接将 $lookup 应用于数组，但是你可以先使用 $unwind 对其进行展开。
没有示例文档，下面的代码片段只是一个通用的方法：
pipeline := []bson.M{
bson.M{"$match": bson.M{"_id": userId }},
bson.M{"$unwind": "$otherUsersIdsArrayName"},
bson.M{
"$lookup": bson.M{
"from": userCollectionName,
"localField": otherUsersIdsArrayName,
"foreignField": "id",
"as": "friend"
}
},
bson.M{"$unwind": "$friend"},
bson.M{
"$group": bson.M{
"_id": "id",
"id": bson.M{"$first": "$id"},
"name": bson.M{"$first": "$name"},
"avatar": bson.M{"$first": "$avatar"},
otherUsersIdsArrayName: bson.M{ "$push": "$friend"}
}
}
}
pipe := collection.Pipe(pipeline)
resp := []bson.M{}
err = pipe.All(&resp)
我应该提到，聚合/管道返回的是 bson.M，而不是已填充的用户对象。毕竟，Mongo 不是关系型数据库。
英文:
You cannot apply $lookup to array directly, but you can to $unwind it first.
Without example documents, the code snippet below is rather a general approach:
pipeline := []bson.M{
bson.M{"$match": bson.M{"_id": userId }},
bson.M{"$unwind": "$otherUsersIdsArrayName"},
bson.M{
"$lookup": bson.M{
"from": userCollectionName,
"localField": otherUsersIdsArrayName,
"foreignField": "id",
"as": "friend"
}
},
bson.M{"$unwind": "$friend"},
bson.M{
"$group": bson.M{
"_id": "id",
"id": bson.M{"$first": "$id"},
"name": bson.M{"$first": "$name"},
"avatar": bson.M{"$first": "$avatar"},
otherUsersIdsArrayName: bson.M{ "$push": "$friend"}
}
}
}
pipe := collection.Pipe(pipeline)
resp := []bson.M{}
err = pipe.All(&resp)
I should mention that aggregation/pipe returns bson.M, not hydrated User objects. Mongo is not a relational database after all.
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