Empty map Ecto behavior

article.ex

defmodule InFactApi.Article do
  use Ecto.Schema
  import Ecto.Changeset
  
  alias InFactApi.{Repo}

  @required_fields [:title, :text]
  @optional_fields [:article_id, :categories, :tags, :authors, :url, :created_date, :raw, :state]

  schema @collection_name do
    field :title
    field :article_id
    field :created_date, Ecto.DateTime
    field :text
    field :state, :string, default: "normal"

    field :url,        {:array, :string}, default: []
    field :categories, {:array, :string}, default: []
    field :authors,    {:array, :string}, default: []
    field :tags,       {:array, :string}, default: []

    field :raw, :map, default: %{}

    timestamps
  end
  
  def changeset(schema, params) do
    cast(schema, params, @required_fields ++ @optional_fields)
  end
end  

existing_articles_mongo.json

[{
    "_id" : ObjectId("577ee03644c6c310ceb59a1d"),
    "article_id" : "3f5ff0c0ca06968af21e5855fb37d79d1bf08a26",
    "authors" : [ 
        "Some Name"
    ],
    "categories" : [ 
        "Category"
    ],
    "inserted_at" : ISODate("2016-07-07T23:05:26.000Z"),
    "raw" : {},
    "tags" : [],
    "text" : "Some text",
    "title" : "Some name",
    "updated_at" : ISODate("2017-02-10T18:28:07.000Z"),
    "url" : [ 
        "http://vesti.ru/"
    ],
    "created_date" : ISODate("2016-04-25T20:45:09.000Z")
}]

sex.sh

iex(1)> InFactApi.Repo.get_by(InFactApi.Article, article_id: "3f5ff0c0ca06968af21e5855fb37d79d1bf08a26", raw: %{})
nil
[debug] QUERY OK db=0.6ms
FIND coll="articles" query=[{"$query", ["$and": [[article_id: "3f5ff0c0ca06968af21e5855fb37d79d1bf08a26"], [raw: []]]]}, {"$orderby", %{}}] projection=%{_id: true, article_id: t
rue, authors: true, categories: true, created_date: true, inserted_at: true, raw: true, state: true, tags: tr
ue, text: true, title: true, updated_at: true, url: true} []