# `Bylaw.Credo.Check.Ecto.PreferRepoAggregateCount`
[🔗](https://github.com/ryanzidago/bylaw/blob/v0.4.0/lib/bylaw/credo/check/ecto/prefer_repo_aggregate_count.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 database-side counting over loading rows with `Repo.all` and counting them
in memory.

`Repo.all(query)` materializes every matching row as an Elixir struct and transfers
all of those rows from the database, even when the caller only needs a number.
`Repo.aggregate(query, :count)` asks the database for the count directly, avoiding
unnecessary row materialization, network transfer, and application memory usage.

Use `Repo.aggregate(query, :count)` when the exact number of matching rows is needed.
Use `Repo.exists?/1` when the caller only needs to know whether at least one row
matches: it expresses that intent directly and lets the database answer an existence
query without counting every matching row.

## Examples

Avoid:

      Repo.all(query) |> Enum.count()
      Enum.count(Repo.all(query))
      query |> Repo.all() |> length()
Prefer:

      Repo.aggregate(query, :count)

Prefer `Repo.exists?/1` or `not Repo.exists?/1` over comparing
`Repo.aggregate(query, :count)` to `0` or `1` for existence checks. Counting is
unnecessary when the result is only used as a boolean, and the existence query can
stop after finding the first matching row.
Avoid:

      Repo.aggregate(query, :count) > 0
      Repo.aggregate(query, :count) == 0
Prefer:

      Repo.exists?(query)
      not Repo.exists?(query)

## Notes

This check uses static AST analysis, so it favors clear source-level patterns over runtime behavior.

## 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.PreferRepoAggregateCount, []}
      ]
    }
  ]
}
```

## 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*