mongodb limit in the embedded document

Question

I need to create a message system, where a person can have a conversation with many users. For example I start to speak with user2, user3 and user4, so anyone of them can see th

Answer 1

The MongoDB docs explain how to select a subrange of an array element.

db.dialogs.find({"_id": [dialogId]}, {msgs:{$slice: 5}}) // first 5 comments
db.dialogs.find({"_id": [dialogId]}, {msgs:{$slice: -5}}) // last 5 comments
db.dialogs.find({"_id": [dialogId]}, {msgs:{$slice: [20, 10]}}) // skip 20, limit 10
db.dialogs.find({"_id": [dialogId]}, {msgs:{$slice: [-20, 10]}}) // 20 from end, limit 10

You can use this technique to only select the messages that are relevant to your UI. However, I'm not sure that this is a good schema design. You may want to consider separating out "visible" messages from "archived" messages. It might make the querying a bit easier/faster.