MongoDb cursor API how to?

What is it you are trying to accomplish?

Thanks a lot for answering!

So, I want pagination in my app. In my website landing page, i would show first 10 items of a search query, then provide buttons to view next / previous 10 items…

I did not find an API that allows me to do that in the driver? E.g., something like: Mongo.find_next(.., cursor, ...)

Or if there is, could u pls show me?

Since this is for a website, you would not have access to the same cursor between requests. For something like this, you would structure your query like

page = 3 # This would be coming in from the URL
limit = 10 # 10 items per page
skip = (page - 1) * limit # This assume your first page is 1, so you will skip 0 for the first page, 10 for the second page, etc
options = [limit: limit, skip: skip, ...] # ... is representing any other options you want to pass in.
Mongo.find(connection, collection, query, options)

Then you are free to render the results as you see fit.

Thanks a lot, I will try this and give feedback…

confirmed that it works! thanks a lot!

Glad I could help!

When use this method with sort: %{<field>: <order>} option, it seems the sorting is not done in the entire result space but only on the returned batch, unless I set page = 0 and limit = <number of all items in DB>

E.g., say I have 10 items in DB labeled 1 → 10. Say an unsorted query, with limit = 10, returns

[1, 3, 2, 4, 8, 5, 9, 7, 6, 10]

Now if I set limit = 5, and sort increasing order, then with page = 0, 1, the two returned batches are:

[1, 2, 3, 4, 8]

and

[5, 6, 7, 9, 10]

What I want are:

[1, 2, 3, 4, 5]

and

[6, 7, 8, 9, 10]

Could u pls show me the way to achieve that, if there is?

Thanks a lot!

I am not seeing the same behaviour.

iex(1)> Mongo.find(pid, "coll", %{}) |> Enum.to_list
[
  %{"_id" => #BSON.ObjectId<5e8f5cd37f1ab13c7b1d99a9>, "coll" => 1},
  %{"_id" => #BSON.ObjectId<5e8f5cd67f1ab13c7b1c6ea3>, "coll" => 2},
  %{"_id" => #BSON.ObjectId<5e8f5cd87f1ab13c7b8e3b4a>, "coll" => 5},
  %{"_id" => #BSON.ObjectId<5e8f5cd97f1ab13c7b686cc8>, "coll" => 3},
  %{"_id" => #BSON.ObjectId<5e8f5cdc7f1ab13c7bdb8c4c>, "coll" => 7},
  %{"_id" => #BSON.ObjectId<5e8f5ce17f1ab13c7b6ebc61>, "coll" => 8},
  %{"_id" => #BSON.ObjectId<5e8f5ce37f1ab13c7bae3a15>, "coll" => 10},
  %{"_id" => #BSON.ObjectId<5e8f5ce57f1ab13c7b2069ea>, "coll" => 9},
  %{"_id" => #BSON.ObjectId<5e8f5cea7f1ab13c7b1f336f>, "coll" => 4},
  %{"_id" => #BSON.ObjectId<5e8f5d0c7f1ab13c7b5b4023>, "coll" => 6}
]
iex(2)> limit = 5
5
iex(3)> page = 1
1
iex(4)> skip = (page - 1) * limit
0
iex(5)> Mongo.find(pid, "coll", %{}, limit: limit, skip: skip, sort: [coll: 1]) |> Enum.to_list
[
  %{"_id" => #BSON.ObjectId<5e8f5cd37f1ab13c7b1d99a9>, "coll" => 1},
  %{"_id" => #BSON.ObjectId<5e8f5cd67f1ab13c7b1c6ea3>, "coll" => 2},
  %{"_id" => #BSON.ObjectId<5e8f5cd97f1ab13c7b686cc8>, "coll" => 3},
  %{"_id" => #BSON.ObjectId<5e8f5cea7f1ab13c7b1f336f>, "coll" => 4},
  %{"_id" => #BSON.ObjectId<5e8f5cd87f1ab13c7b8e3b4a>, "coll" => 5}
]
iex(6)> page = 2
2
iex(7)> skip = (page - 1) * limit # Make sure you are setting the proper value for skip each time.
5
iex(8)> Mongo.find(pid, "coll", %{}, limit: limit, skip: skip, sort: [coll: 1]) |> Enum.to_list
[
  %{"_id" => #BSON.ObjectId<5e8f5d0c7f1ab13c7b5b4023>, "coll" => 6},
  %{"_id" => #BSON.ObjectId<5e8f5cdc7f1ab13c7bdb8c4c>, "coll" => 7},
  %{"_id" => #BSON.ObjectId<5e8f5ce17f1ab13c7b6ebc61>, "coll" => 8},
  %{"_id" => #BSON.ObjectId<5e8f5ce57f1ab13c7b2069ea>, "coll" => 9},
  %{"_id" => #BSON.ObjectId<5e8f5ce37f1ab13c7bae3a15>, "coll" => 10}
]

Would you be able to show something similar to what I just did there? If you can, I may be able to help figure out what is going wrong.

hmm I noticed that, for every returned batch, if I do Enum.reverse the list, then the order is correct as I wanted.

Sorry I don’t have an example right now and cannot share data, but here is two snippets with the later does what I want:

   order = cond do
      sort_type == Const.sort_inc -> 1
      sort_type == Const.sort_dec -> -1
      true                        -> -1
    end
    sort = cond do
      sort_by == Const.rating_sort -> [rating:     order]
      sort_by == Const.price_sort  -> [price_out:  order]
      sort_by == Const.time_sort   -> [updated_at: order]
      true                         -> [rating:     order]
    end
   opts  = [ 
      limit: batch_size,
      skip:  (batch - 1) * batch_size,
      sort:  sort ]
    ItemM |> DbSrv.query(q, opts)

and

   order = cond do
      sort_type == Const.sort_inc -> 1
      sort_type == Const.sort_dec -> -1
      true                        -> -1
    end
    sort = cond do
      sort_by == Const.rating_sort -> [rating:     order]
      sort_by == Const.price_sort  -> [price_out:  order]
      sort_by == Const.time_sort   -> [updated_at: order]
      true                         -> [rating:     order]
    end
   opts  = [ 
      limit: batch_size,
      skip:  (batch - 1) * batch_size,
      sort:  sort ]
    ItemM |> DbSrv.query(q, opts)
               |> Enum.reverse

The DbSrv module is just a convenient wrapper around Mongo module of yours.

My bad, I did an Enum.reduce in the DbSrv module! Sorry for ur time! Everything is fine now.

Thanks a lot!