在mongodb聚合中包含字段 [英] Include fields in mongodb aggregate

问题描述

我有以下收藏:

{"orderID" : "30688", "branch" : "CO", "customerID" : "11396783", "customerEmail" : "[e-mail]"}
{"orderID" : "30688", "branch" : "CO", "customerID" : "11396783", "customerEmail" : "[e-mail]"}
{"orderID" : "30688", "branch" : "CO", "customerID" : "11396783", "customerEmail" : "[e-mail]"}
{"orderID" : "89765", "branch" : "CO", "customerID" : "54157526", "customerEmail" : ""}
{"orderID" : "89765", "branch" : "CO", "customerID" : "54157526", "customerEmail" : ""}
{"orderID" : "21546", "branch" : "CO", "customerID" : "20103585", "customerEmail" : "[e-mail]"}
{"orderID" : "21546", "branch" : "CO", "customerID" : "20103585", "customerEmail" : "[e-mail]"}
{"orderID" : "21546", "branch" : "KA", "customerID" : "89374792", "customerEmail" : "[e-mail]"}
{"orderID" : "21794", "branch" : "NY", "customerID" : "78125522", "customerEmail" : ""}

我需要在customerEmail不为null的某个分支中获取所有唯一的客户ID.我对分支"的期望:"CO"

I need to get all unique customerIDs in a certain branch whose customerEmail is not null. What I expect for "branch":"CO"

{"customerID" : "11396783", "customerEmail" : "[e-mail]"}
{"customerID" : "20103585", "customerEmail" : "[e-mail]"}

到目前为止，我已经尝试过:

So far I've tried:

db.collection.aggregate([
    { $match: { branch: "CO" } },   
    { $group:
        {
            _id: { customer:"$customerID"}
        }
    },
    {
        $group: {_id:"$_id.customer"}
    },
    {
        $addFields: {  email: "$customerEmail"} 
    }
]);

但它不带电子邮件字段.

but it doesn't bring the email field.

推荐答案

它不包含该字段，因为您没有要求该字段返回.您在这里缺少的是使用 $first 或类似的累加器" ，以便在 $group .

It does not include the field because you did not ask for the field to return. The thing you are missing here is using $first or a similar "accumulator" in order to return the element during the $group.

如果您不想使用空的电子邮件地址，也可以将其排除在

Also if you don't want the empty email address then exclude it within the $match pipeline stage, since that's the most efficient thing to do.

db.collection.aggregate([
    { $match: { branch: "CO", "customerEmail": { "$ne": "" } } },
    { $group:
        {
            _id: { customer:"$customerID"},
            email: { "$first": "$customerEmail" }
        }
    }
]);

管道"仅从类似 或您实际要求的 $project 它来.就像"Unix管道" |运算符一样，下一个阶段"可用的唯一内容就是输出的内容.

A "pipeline" only returns "output" from stages like $group or $project that you actually ask it to. Just like the "Unix pipe" | operator, the only things available to the "next stage" are what you output.

这应该从以下内容中显而易见:

This should be evident simply from:

db.collection.aggregate([
    { $match: { branch: "CO" } },   
    { $group:
        {
            _id: { customer:"$customerID"}
        }
    }
]);

甚至:

db.collection.aggregate([
    { $match: { branch: "CO" } },   
    { $project:
        {
            _id: { customer:"$customerID"}
        }
    }
]);

当然，其中只返回_id值，因为这是您所要求的.

Which returns of course only the _id value since that is all you asked for.

您只能在任何管道阶段中访问上一阶段输出"的数据.在 $group 中，这意味着仅_id分组密钥，以及使用有效的明确" 指定的任何内容="nofollow noreferrer">累加器" ，以获取您希望返回的任何其他属性.任何累加器(在此对字符串"有效)都可以，但是_id 必须之外的任何内容都必须使用.

You only have access in any pipeline stage to the data which was "output by the previous stage". Within a $group that means only the _id for the grouping key, and whatever was specified "explicitly" using a valid "accumulator" for any other properties you wish to return. Any accumulator ( which is valid for a "string" here ) will do, but anything outside of the _id must use an "accumulator".

我建议花点时间看一下所有聚合运算符以及他们的实际工作.每个运算符都有示例用法

I suggest taking the time to look at all the aggregation operators and what they actually do. There is example usage with each operator

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