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Connecting Ecto to a legacy database in a script

Written June 22, 2020. Tagged Elixir, Ecto.

I wanted to use Ecto in a script to read data from a legacy database, transform the data, and stick it in the current database.

The documentation focuses mostly on how to use Ecto with app-level configuration, in config/config.exs and friends.

But I didn't want to add app-level configuration for this one-off script. I just wanted to pass in the database URL.

After some experimentation, this works with Ecto 3.4:

old_db_url = System.get_env("OLD_DB_URL") || raise("Missing OLD_DB_URL!")

defmodule OldRepo do
use Ecto.Repo,
otp_app: :my_app,
adapter: Ecto.Adapters.Postgres,
read_only: true # Let's be safe, if we don't need to write.

# This bit is completely optional.
# You can skip it and do schemaless queries.
defmodule OldItem do
use Ecto.Schema

schema "items" do
field :name, :string
# and so on

OldRepo.start_link(url: old_db_url, ssl: true)

# Verify it works, if you defined a schema.
IO.inspect count: OldRepo.aggregate(OldItem, :count)

# Verify it works, if you're schemaless.
IO.inspect count: OldRepo.aggregate("items", :count)

You run the script with

OLD_DB_URL="postgres://secretsauce…" mix run priv/repo/migrate_data.exs

in a terminal.

In my case, I wanted to run the code from my local machine but against production databases. So I added a new_db_url and a NewRepo to the script as well, but used the schemas I already had in the app with that repo – e.g. MyApp.Item.

If I had been running the script in production, or wanted to copy to the development database, I could have used MyApp.Repo instead of creating a NewRepo.

Anyway, that's it! Not a lot of code, but took me a little while to piece together.