[{"images":[{url:"img1"},{url:"img2"}],posts:[{description:"post1",images:[{url:"post_img11"},{url:"post_img12"}]},{description:"post2",images:[{url:"post_img21"},{url:"post_img22"}]}]}]

db.collection.aggregate([{$set:{combinedImages:{"$concatArrays":["$images",{$reduce:{input:"$posts",initialValue:[],in:{"$concatArrays":["$$value","$$this.images"]}}}]}}}])