MongoDb aggregation-framework, join different documents are in the same collection -

i want mongodb 1 of several documents. have following data structure:

db.coll.insert([  {_id: "gomer", doc_type: "user", group: ["user", "author"] },  {_id: "dante", doc_type: "user", group: ["user"] },  {_id: 1, doc_type: "blogs", user: "gomer", article: "aaaa" },  {_id: 2, doc_type: "blogs", user: "vasya", article: "bbbb" } ]) 

i want result of request of joint document:

{ _id:"123",    blogs: {id:1, doc_type: "blogs", user: "gomer", article: "aaaa"},    user : {id:"gomer", doc_type: "user", group: ["user", "author"]}  } 

but can not write valid request:

db.coll.aggregate([   { $match:   {$or:[{doc_type:"blogs"},{doc_type:"user"}]} },   { $project: {       blogs: { id:$id, doc_type:"blogs", user:$user, article:$article },       user:  { id:$id, doc_type:"user",  group:$group  }         }     } ]) 

how make request?

i got join several documents 1 query. all, got answer question.

db.test.aggregate([     { $match:  { $or: [ {doc_type: "blogs"}, {doc_type: "user"} ] } },     { $project: {              a: 1,             blogs: {                 $cond: {                     if: { doc_type: '$blogs'},                     then: {_id:"$_id", user:"$user", article:"$article"},                     else: null                 }             },             user: {                 $cond: {                     if: { doc_type: '$user' },                     then: { _id:"$_id", group:"$group"},                     else: null                 }             }         }     },     { $group : {                _id : { a: "$a" },                user: { $push: "$user" },                blog: { $push: "$blogs" },             }     },     { $unwind : "$blog" },     { $unwind : "$user" },     { $project:{          user: "$user",         article: "$blog",         matches: { $eq:[ "$user._id", "$blog.user" ] } }     },      { $match: { matches: true } } ])