What's the $unwind operator in MongoDB?

Question

This is my first day with MongoDB so please go easy with me :)

I can\'t understand the $unwind operator, maybe because English is not my native language

Answer 1

Let me explain in a way corelated to RDBMS way. This is the statement:

db.article.aggregate(
    { $project : {
        author : 1 ,
        title : 1 ,
        tags : 1
    }},
    { $unwind : "$tags" }
);

to apply to the document / record:

{
 title : "this is my title" ,
 author : "bob" ,
 posted : new Date () ,
 pageViews : 5 ,
 tags : [ "fun" , "good" , "fun" ] ,
 comments : [
             { author :"joe" , text : "this is cool" } ,
             { author :"sam" , text : "this is bad" }
 ],
 other : { foo : 5 }
}

The $project / Select simply returns these field/columns as

SELECT author, title, tags FROM article

Next is the fun part of Mongo, consider this array tags : [ "fun" , "good" , "fun" ] as another related table (can't be a lookup/reference table because values has some duplication) named "tags". Remember SELECT generally produces things vertical, so unwind the "tags" is to split() vertically into table "tags".

The end result of $project + $unwind:

Translate the output to JSON:

{ "author": "bob", "title": "this is my title", "tags": "fun"},
{ "author": "bob", "title": "this is my title", "tags": "good"},
{ "author": "bob", "title": "this is my title", "tags": "fun"}

Because we didn't tell Mongo to omit "_id" field, so it's auto-added.

The key is to make it table-like to perform aggregation.