Monthly Downloads: 7,734
Programming language: Elixir
License: Apache License 2.0
Latest version: v0.5.1

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Build Status

Documentation for MongoDB is available online.


  • Supports MongoDB versions 3.4, 3.6, 4.0
  • Connection pooling (through db_connection)
  • Streaming cursors
  • Performant ObjectID generation
  • Follows driver specification set by 10gen
  • Safe (by default) and unsafe writes
  • Aggregation pipeline
  • Replica sets
  • Sessions and transactions

Immediate Roadmap

  • Make sure requests don't go over the 16 MiB limit
  • New 2.6 write queries and bulk writes

Tentative Roadmap

Data representation

BSON Elixir
double 0.0
string "Elixir"
document [{"key", "value"}] \
binary %BSON.Binary{binary: <<42, 43>>, subtype: :generic}
object id %BSON.ObjectId{value: <<...>>}
boolean true
UTC datetime %DateTime{}
null nil
regex %BSON.Regex{pattern: "..."}
JavaScript %BSON.JavaScript{code: "..."}
integer 42
symbol "foo" ²
min key :BSON_min
max key :BSON_max

¹ Since BSON documents are ordered Elixir maps cannot be used to fully represent them. This driver chose to accept both maps and lists of key-value pairs when encoding but will only decode documents to maps. This has the side-effect that the information about order of keys in a BSON document is lost when it's decoded. Additionally the driver will accept both atoms and strings for document keys but will only decode to strings.

² BSON symbols can only be decoded.

Writing your own encoding info

If you want to write a custom struct to your mongo collection - you can do that by implementing Mongo.Encoder protocol for your class. The output should be a map, which will be passed to the Mongo database.


defmodule CustomStruct do
  @fields [:a, :b, :c, :id]
  @enforce_keys @fields
  defstruct @fields

  defimpl Mongo.Encoder do
    def encode(%{a: a, b: b, id: id}) do
        _id: id,
        a: a,
        b: b,
        custom_encoded: true

So, given the struct:

%CustomStruct{a: 10, b: 20, c: 30, id: "5ef27e73d2a57d358f812001"}

it will be written to database, as:

  "a": 10,
  "b": 20,
  "custom_encoded": true,
  "_id": "5ef27e73d2a57d358f812001"



Add mongodb to your mix.exs deps.

defp deps do
    {:mongodb, "~> 0.5.1"}

Then run mix deps.get to fetch dependencies.

Connection pooling

By default mongodb will start a single connection, but it also supports pooling with the :pool_size option.

# Starts an unpooled connection
{:ok, conn} = Mongo.start_link(url: "mongodb://localhost:27017/db-name")

# Gets an enumerable cursor for the results
cursor = Mongo.find(conn, "test-collection", %{})

|> Enum.to_list()
|> IO.inspect

If you're using pooling it is recommend to add it to your application supervisor:

def start(_type, _args) do
  children = [
    {Mongo, [name: :mongo, database: "test", pool_size: 2]}

  opts = [strategy: :one_for_one, name: MyApp.Supervisor]
  Supervisor.start_link(children, opts)

Simple start with pooling:

{:ok, conn} = Mongo.start_link(name: :mongo, database: "test", pool_size: 2)

Operate the mongodb with specify pool name in each query:

Mongo.find(:mongo, "collection", %{}, limit: 20)

More pool options in here.

Replica Sets

To connect to a MongoDB cluster that is using replica sets, it is recommended to use the :seeds list instead of a :hostname and :port pair.

{:ok, pid} = Mongo.start_link(database: "test", seeds: ["hostname1.net:27017", "hostname2.net:27017"])

This will allow for scenarios where the first "hostname1.net:27017" is unreachable for any reason and will automatically try to connect to each of the following entries in the list to connect to the cluster.

Auth mechanisms

For versions of MongoDB 3.0 and greater, the auth mechanism defaults to SCRAM. If you'd like to use MONGODB-X509 authentication, you can specify that as a start_link option.

{:ok, pid} = Mongo.start_link(database: "test", auth_mechanism: :x509)

AWS, TLS and Erlang SSL ciphers

Some MongoDB cloud providers (notably AWS) require a particular TLS cipher that isn't enabled by default in the Erlang SSL module. In order to connect to these services, you'll want to add this cipher to your ssl_opts:

{:ok, pid} = Mongo.start_link(database: "test",
      ssl_opts: [
        ciphers: ['AES256-GCM-SHA384'],
        cacertfile: "...",
        certfile: "...")


Using $and

Mongo.find(:mongo, "users", %{"$and" => [%{email: "[email protected]"}, %{first_name: "first_name"}]})

Using $or

Mongo.find(:mongo, "users", %{"$or" => [%{email: "[email protected]"}, %{first_name: "first_name"}]})

Using $in

Mongo.find(:mongo, "users", %{email: %{"$in" => ["[email protected]", "[email protected]"]}})


The SSL test suite is enabled by default. You have two options. Either exclude the SSL tests or enable SSL on your MongoDB server.

Disable the SSL tests

mix test --exclude ssl

Enable SSL on your Mongo server

$ openssl req -newkey rsa:2048 -new -x509 -days 365 -nodes -out mongodb-cert.crt -keyout mongodb-cert.key
$ cat mongodb-cert.key mongodb-cert.crt > mongodb.pem
$ mongod --sslMode allowSSL --sslPEMKeyFile /path/to/mongodb.pem
  • For --sslMode you can use one of allowSSL or preferSSL
  • You can enable any other options you want when starting mongod


Copyright 2015 Justin Wood and Kobil Systems GmbH

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at


Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.

*Note that all licence references and agreements mentioned in the mongodb README section above are relevant to that project's source code only.