need trello mongodb schema

So I've been trying to find the schemas that trello use for their backend. I have an assignment in my backend course to create a trello clone but with less features, now all i want to know is if they use arrays to keep references
(Example: boardSchema {title: string,
Lists[
mongoose.Types.ObjectId,
ref: List
]}) or if they only keep a reference to the parent
(Example:
ListSchema {
Title: string
BoardId:
mongoose.Type.ObjectId,
Ref: Board
})

Anybody got any ideas?