# `Bylaw.Credo.Check.Ecto.PreferRepoOneOverAllFirst`
[🔗](https://github.com/ryanzidago/bylaw/blob/v0.4.0/lib/bylaw/credo/check/ecto/prefer_repo_one_over_all_first.ex#L1)

## Basics

> #### This check is disabled by default. {: .neutral}
>
> [Learn how to enable it](`e:credo:config_file.html#checks`) via `.credo.exs`.

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`:

```elixir
%{
  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](`e:credo:check_params.html`) can be applied.

Parameters can be configured via the [`.credo.exs` config file](`e:credo:config_file.html`).

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*Consult [api-reference.md](api-reference.md) for complete listing*