Get all assignments given a user's ID

mongocommand

db.courses.aggregate([{
    $match: {
        students: "52868c8f38f0777b4e000007"
      }
    },
    {
      $unwind: "$assignments"
    },
    {
    $group: {
        _id: "$assignments"
      }
    },
    {
      $project: {
        assignment: "$_id",
        _id: false
      }
    }
]);

result

{
	"result" : [
		{
			"assignment" : {
				"_id" : ObjectId("529d0a17c015167c71000007"),
				"title" : "Chapter 8",
				"posted" : 1386023447107,
				"due" : 1386046800000,
				"description" : "New assignment for chapter 8"
			}
		},
		{
			"assignment" : {
				"_id" : ObjectId("528e20c37973fe8b5e000007"),
				"title" : "Zac",
				"posted" : 1385046211646,
				"due" : 1385046205813,
				"description" : "asdfaadsa"
			}
		},
		{
			"assignment" : {
				"_id" : ObjectId("528a8fb6526c429a2e00000b"),
				"title" : "Nick's assignment",
				"posted" : 1384812470768,
				"due" : 1384812464060,
				"description" : "asdfasdfasdf"
			}
		},
		{
			"assignment" : {
				"_id" : ObjectId("528a8f7d526c429a2e00000a"),
				"title" : "Mike's assignment",
				"posted" : 1384812413179,
				"due" : 1385355600000,
				"description" : "asdfasdfadsfs"
			}
		}
	],
	"ok" : 1
}