[{author:"ABC1",text:"this is a Post",details:{time:"14/05/2015",Edit:"none"},comments:[{comment_text:"Hello",user:"alan",time:"20/05/2014 20:44"},{comment_text:"Hi Every One",user:"bob",time:"20/05/2014 20:44"},{comment_text:"Good morning , Alan",user:"Alan",time:"20/05/2014 20:44"},{comment_text:"I'm bob",user:"bob",time:"20/05/2014 20:44"},],category:"IT"}]

db.collection.aggregate([{$project:{_id:0,author:1,text:1,details:1,category:1,comments:{$filter:{input:"$comments",as:"comment",cond:{$eq:["$$comment.user","bob"]}}}}}])