Bylaw.Credo.Check.Ecto.PreferRepoOneOverAllFirst (bylaw_credo v0.4.0)

Basics

This check has a base priority of high and works with any version of Elixir.

Explanation

Prefer a single-row Repo read over loading every matching row and taking the first result in Elixir.

Repo.all retrieves and materializes every match even when the caller needs only one row. Repo.one expresses the cardinality expectation at the Repo boundary, avoids unnecessary row transfer, and can surface an unexpected second match instead of silently discarding it.

Examples

Avoid:

query
|> Repo.all()
|> List.first()

query
|> Repo.all()
|> Enum.at(0)

query
|> Repo.all()
|> hd()

When the query is expected to return zero or one row, prefer:

Repo.one(query)

When the query intentionally selects the first row from an ordered result, preserve that intent in the query:

query
|> Ecto.Query.first()
|> Repo.one()

Use the bang variants when a missing row is exceptional. For primary-key lookups, prefer Repo.get/2 or Repo.get!/2.

Notes

This check uses static AST analysis. It reports direct Repo.all and first-element selection with List.first/1, Enum.at/2 with the literal index 0, or hd/1, including piped forms. It cannot infer whether the caller expects uniqueness or an ordered first row. Other Enum.at/2 indices are outside this check's scope because they express different offset and ordering semantics.

Options

This check has no check-specific options. Configure it with an empty option list.

Usage

Add this check to Credo's checks: list in .credo.exs:

%{
  configs: [
    %{
      name: "default",
      checks: [
        {Bylaw.Credo.Check.Ecto.PreferRepoOneOverAllFirst, []}
      ]
    }
  ]
}

Check-Specific Parameters

There are no specific parameters for this check.

General Parameters

Like with all checks, general params can be applied.

Parameters can be configured via the .credo.exs config file.