How to return just the nested documents of an array from all documents

Question

I have a question about querying nested documents. I tried to search but nothing answered my question or I am maybe overlooking it. I have structure like this:

Answer 1

According to above mentioned description please try executing following query in MongoDB shell.

db.collection.aggregate(

    // Pipeline
    [
        // Stage 1
        {
            $unwind: "$books"
        },

        // Stage 2
        {
            $group: {
              _id:null,
              books:{$addToSet:'$books'}
            }
        },

        // Stage 3
        {
            $project: {
                books:1,
                _id:0
            }
        },

    ]

);