MongoDb cursor API how to?

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1

Hi all,

I am new to Elixir and are using this lib https://github.com/ankhers/mongodb to work with MongoDB. The doc says it has “Streaming cursor” feature, but the source code implements only Enumerable protocol for the cursor returned by the Mongo.find function.

There is no doc about a cursor API, or do I miss something?

Thanks a lot!

2

What is it you are trying to accomplish?

3

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?

4

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.

1 Like
5

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

6

confirmed that it works! thanks a lot!

7

Glad I could help!

8

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!

1 Like
9

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.

1 Like
10

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.

11

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

Thanks a lot!

12

Thanks guys