The next step after scaffolding my first Phoenix project was to populate my database with data representing French departments (departments are administrative divisions, we have 101 of them) and communes (communes are the smallest administrative divisions, we have 36,681 of them!).

Forewords

The CSV I import is from open data shared by the French government. The goal is to read this big (24MB) CSV file and insert records for departments and communes not already registered in the database.

I’m using Phoenix v1.1.0 and Ecto v1.1.0 in the rest of the article.

A big thanks to José Valim for proposing an optimization to my code (included in the rest of the article)!

Here are the steps we will follow to import data from the CSV to our database:

  1. Taming the streaming beast
  2. Working with Ecto
  3. Having fun with an agent

### 1. Taming the streaming beast

The first thing I realized was that it would have been inefficient to read the whole file, store its content in memory and loop over it. That’s why I started digging through the Stream module.

After some trial and errors, I stumbled upon the CSV module which gives the following sample code:

File.stream!("data.csv") |>
CSV.decode(separator: ?\t) |>
Enum.map fn row ->
  Enum.each(row, &String.upcase/1)
end

Since my CSV file uses ; as separator and includes the headers in its first row, I’ve adapted a bit the snippet to my use-case, as follow:

defmodule Seeds.DepartmentsAndCommunes do

  @doc "Imports departments and communes from the given CSV to the database"
  def import_from_csv(csv_path) do
    Agent.start_link(fn -> %{departments: %HashDict{}, communes: []} end, name: __MODULE__)

    File.stream!(Path.expand(csv_path))
    |> CSV.decode(separator: ?;, headers: true)
    |> Stream.each(fn row ->
      _process_csv_row(row, agent)
    end)
    |> Stream.run
  end

  defp _process_csv_row(row, agent) do
    # TODO
  end

end

Note:

  • I put this code into priv/repo/seeds.exs so I can run it with mix run priv/repo/seeds.exs;
  • I create an agent to store the already processed departments/communes to avoid unnecessary DB queries. More details in the “Having fun with an agent” part.

### 2. Working with Ecto

The second step is the actual work to populate the database.

Following are the functions needed to insert/update a department into the database:

@doc "Inserts or updates a DB record for the given department params that must contain the :name and :insee_code keys"
defp _process_department(department_params) do
  # Let's avoid a DB query if this department has already been processed
  unless _get_processed(:departments)[department_params[:name]] do
    # Otherwise, try to fetch the department from the DB or instantiate an empty record
    department = case Repo.get_by(Department, insee_code: department_params[:insee_code]) do
      nil        -> %Department{} # Department not found, we build one
      department -> department    # Post exists, let's use it
    end
    # Create a changeset
    |> Department.changeset(department_params)
    # Take advantage of `insert_or_update!` from Ecto 1.1
    |> Repo.insert_or_update!

    _add_processed(:departments, department_params[:name], department.id)
  end
end

Similarly, here is the function to insert/update a commune:

@doc "Inserts or updates a DB record for the given commune params that must contain the :name, :commune_code and :department_id keys"
defp _process_commune(commune_params) do
  unless Enum.member?(_get_processed(:communes), commune_params[:name]) do
    commune = case Repo.get_by(Commune, commune_code: commune_params[:commune_code]) do
      nil     -> %Commune{} # Commune not found, we build one
      commune -> commune    # Post exists, let's use it
    end
    |> Commune.changeset(commune_params)
    |> Repo.insert_or_update!

    _add_processed(:communes, commune.name)
  end
end

### 3. Having fun with an agent

To avoid doing two queries per CSV row (one for the department, one for the commune), I needed to store the already processed departments/communes. I think an Elixir Agent is well-suited for that since it’s a simple abstraction around state.

As we already saw, I have instantiated an agent with:

Agent.start_link(fn -> %{departments: %HashDict{}, communes: []} end, name: __MODULE__)

Then I have created a bunch of shortcut functions to access and update the state:

defp _get_processed(:departments) do
  Agent.get(__MODULE__, fn %{departments: departments} -> departments end)
end

@doc "Returns the already-processed communes names List"
defp _get_processed(:communes) do
  Agent.get(__MODULE__, fn %{communes: communes} -> communes end)
end

@doc "Adds a {name => db_id} tuple in the already-processed departments HashDict"
defp _add_processed(:departments, name, department_id) do
  Agent.update(__MODULE__, fn %{departments: departments} = processed ->
    %{processed | departments: Dict.put(departments, name, department_id)}
  end)
end

@doc "Adds a commune name tuple in the already-processed communes List"
defp _add_processed(:communes, name) do
  Agent.update(__MODULE__, fn %{communes: communes} = processed ->
    %{processed | communes: [name | communes]}
  end)
end

Conclusion

That’s it! You can find the whole script in this GitLab snippet.

I really struggled with the “memoization” of the already-processed departments & communes at first: I was passing two arrays all around the place, I didn’t understand how to update them etc.

Then I found this nice “How I Start” article (with José Valim) which reminded me of the Agent module, why it’s useful and how to actually use it.

I must admit, that was an awesome “Eurêka” moment!

Regarding the next steps for this script, here is what I have in mind:

  • Fetch the latest CSV file from the data.gouv.fr API and process it;
  • Do this regularly in a background worker.

What about you, have you written similar scripts with Elixir before? Also, if you have ideas on how to improve my script, I’m all hears (hint: you can open a pull-request below)!