Ecto is a data validation and database persistence elixir library. This is the backbone of database operations in the Phoenix Webframework.
Transactions in Ecto
Ecto well supports database transactions via the Multi module. You can setup arbitrary functions and run them in sequence with good error handling support1:
step_1 = fn repo, _ ->
%Dog{name: "hello"}
|> repo.insert()
end
step_2 = fn repo, %{step_1_call: dog} ->
%DogCollar{dog_id: dog.id}
|> Repo.insert
end
Multi.new
|> Multi.run(:step_1_call, step_1)
|> Multi.run(:step_2_call, step_2)
|> Repo.transaction
Notice that the results from the first call are packed into the map argument to the function of the next call under the name that the step was run (:step_1_call).
Partial Unique Index (Boolean column) in Ecto Migration
It’s useful to be able to say that for a given column pair, there can be only one of one kind of a pair of attributes. For example, consider a team has many mambas but it can only have one captain.
Assume a structure like
defmodule Player do
schema "players" do
field :name, :string
field :is_captain, :boolean, default: false
belongs_to :team, Team
timestamps()
end
end
defmodule Team do
schema "teams" do
field :name, :string
has_many :players, Player
timestamps()
end
end
Now we want to enforce that for a given, there is only one player at a time who can be the captain (is_captain true).
In the migration that creates the players:
create table(:players) do
add :is_captain, :boolean, default: false, null: false
add :name, :string
add :team_id, references(:teams, on_delete: :nothing)
timestamps()
end
create index(:players, [:team_id])
create unique_index(:players, [:team_id, :is_captain],
name: "player_team_captain_index",
where: "is_captain IS TRUE"
)
Importantly is the where clause on the unique index which makes the constrain unique for team id where the is_captain is true. Note that you cannot just do where: is_captain because the index definition is finicky in postgres. Also, is_captain = true will not work.
Finally, you can setup the changeset to properly catch this uniqueness conflict:
def changeset(player, attrs) do
player
|> cast(attrs, [:name, :team_id, :is_captain])
|> cast_assoc(:team)
|> unique_constraint([:team_id, :is_captain], name: "player_team_captain_index")
end