Insert many json file inside one document in mongoDB

i have a question: but how can i insert many json files inside one document collection? I have a ruby script connected with mongoDB which generate json files for each ID product. In mongo i should want a structure like this:

Id(document's name) : {

many json for same ID

} 

how can i get this structure in ruby?

DB's name is "test_db" and collection's name is "test_coll"