Filtering an array inside an array in Mongo DB

Question

I want to filter an array inside an array in Mongo DB. For example, given these 2 documents:

{ 
"_id" : 1.0, 
"pages" : [
    {
        "texts" : [
            {
                "text" : "foo"
            }, 
            {
                "text" : "foo1"
            }, 
            {
                "text" : "foo2"
            }
        ]
    }
]
}
{ 
"_id" : 2.0, 
"pages" : [
    {
        "texts" : [
            {
                "text" : "bar"
            }, 
            {
                "text" : "bar1"
            }, 
            {
                "text" : "bar2"
            }
        ]
    }
]
}

I want a query that finds the _id and text given the text as input, for example, for the keyword "bar1" the output would be something like this:

{ 
"_id" : 2.0, 
"text": "bar1"
}

I can achieve this if the text that I am looking for is in a single array at the top level of the document, but I cannot do it with an array inside another array.

So far my query looks like this:

db.collection.aggregate([{$match:{"pages.texts.text":"bar1"}},
                { $project:
                    { text: 
                        { $filter: 
                            { input: '$pages.texts.text', 
                              as: 'text', 
                              cond: { $eq: [ "$$text" , "bar1"]}
                            }
                        },
                      _id:1
                    }
                }
                ])

Which gives as output this:

{ 
"_id" : 2.0, 
"text" : [

]
}

It shows the correct _id but no text.

If I change the operator $eq for $ne in the filter the output is:

{ 
"_id" : 2.0, 
"text" : [
    [
        "bar", 
        "bar1", 
        "bar2"
    ]
]
}

It seems that text is an array of arrays and I cannot compare directly the keyword with this field. I have tried different approaches to solve this but no luck so far.

Any help would be greatly appreciated.

Thank you!

Answer 1

Since you only want the (double) nested element that matches your search, you can $unwind both arrays first, then just use $match to get to the element you want:

[
    {
        $unwind: 
            "$pages"

    },
    {
        $unwind: 
            "$pages.texts"

    },
    {
        $match:{
            "pages.texts.text":"bar1"
        }
    },
    {
        $project: {
            _id:true,
            text: "$pages.texts.text"
        }
    }
]