Mongodb regex in aggregation using reference to field value

问题

note: I'm using Mongodb 4 and I must use aggregation, because this is a step of a bigger aggregation

Problem

How to find in a collection documents that contains fields that starts with value from another field in same document ?

Let's start with this collection:

db.regextest.insert([
{"first":"Pizza", "second" : "Pizza"},
{"first":"Pizza", "second" : "not pizza"},
{"first":"Pizza", "second" : "not pizza"}
])

and an example query for exact match:

db.regextest.aggregate([
{
    $match :  { $expr: { $eq: [ "$first" ,"$second" ] }   }    }
])

I will get a single document

{
    "_id" : ObjectId("5c49d44329ea754dc48b5ace"),
    "first" : "Pizza",    "second" : "Pizza"
}

And this is good.

But how to do the same, but with startsWith ? My plan was to use regex but seems that is not supported in aggregation so far.

With a find and a custom javascript function works fine:

db.regextest.find().forEach(
    function(obj){
        if (obj.first.startsWith(obj.second)){
            print(obj);
        }
    }
)

And returns correctly:

{
    "_id" : ObjectId("5c49d44329ea754dc48b5ace"),
    "first" : "Pizza",
    "second" : "Pizza"
}

How it's possible to get same result with aggregation framework ?

One idea is to use existing aggregation framework pipeline, out to a temp colletion and then run the find above, to get match I'm looking for. This seems to be a workaround, I hope someone have a better idea.

Edit: here the solution

db.regextest.aggregate([{
        $project : {
            "first" : 1,
            "second" : 1,
            fieldExists : {
                $indexOfBytes : ['$first', '$second' , 0]
            }
        }
    }, {
        $match : {
            fieldExists : {
                $gt : -1
            }
        }
    }
]);

回答1:

The simplest way is to use $expr first available in 3.6 like this:

{$match:{$expr:{$eq:[
    “$second”,
    {$substr:{
        “$first”,
        0,
        {$strLenCP:”$second”}
    }}
]}}}

This compare the string in field ‘second’ with the first N characters of ‘first’ where N is the length of second string. If they are equal, then ‘first’ starts with ‘second’.

4.2 adds support for $regex in aggregation expressions, but starts with is much simpler and doesn’t need regular expressions.

来源：https://stackoverflow.com/questions/54365355/mongodb-regex-in-aggregation-using-reference-to-field-value

标签

mongodb

aggregation-framework