# `Bylaw.Ecto.Query.Checks.DateDatetimeMixedComparisons`
[🔗](https://github.com/ryanzidago/bylaw/blob/v0.1.0-alpha.1/lib/bylaw/ecto/query/checks/date_datetime_mixed_comparisons.ex#L1)

Validates that date fields are not compared to datetime fields without explicit truncation.

This catches queries where a field backed by `:date` is compared directly to
a field backed by `:utc_datetime`, `:utc_datetime_usec`, `:naive_datetime`, or
`:naive_datetime_usec`.

## Examples

Bad:

    from(Event, as: :event)
    |> where([event: e], e.event_date <= e.inserted_at)

Why this is bad:

PostgreSQL can implicitly cast across date and timestamp types. With UTC
timestamp fields, that cast may depend on the database session timezone. The
date boundary is not visible in the query.

Better:

    from(Event, as: :event)
    |> where([event: e], e.event_date <= type(e.inserted_at, :date))

Why this is better:

The datetime side is explicitly treated as a date, so the boundary decision is
visible to reviewers.

## Notes

This check inspects supported direct field-to-field comparisons and `in`
predicates. It ignores values, schema-less sources, hidden fragment access,
and subqueries.

## Options

  * `:validate` - explicit `false` disables the check. Defaults to `true`.

The check is static. It inspects direct field-to-field comparisons and `in`
predicates in `where`, `having`, and direct join `on` predicates, reflecting
field types from the root schema, direct explicit join schemas, and
association join schemas when the owner binding schema is known. It detects
dot field access, `field/2`, named bindings, dynamic predicates, and
`type(field, :date)` wrappers used as explicit truncation. It ignores values,
schema-less sources, fragments that hide field access, and subqueries.

## Usage

Add this module to the explicit check list passed through `Bylaw.Ecto.Query`.
See `Bylaw.Ecto.Query` for the full `c:Ecto.Repo.prepare_query/3` setup.

# `validate`

```elixir
@spec validate(
  Bylaw.Ecto.Query.Check.operation(),
  Bylaw.Ecto.Query.Check.query(),
  opts()
) ::
  Bylaw.Ecto.Query.Check.result()
```

Implements the `Bylaw.Ecto.Query.Check` validation callback.

---

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