Joined tables add all columns to column set

[jdmcd]

Describe the bug

When running a Fluent query with the following join:

let courseSections = try await CourseSection.query(on: conn)
    .join(DistrictCourse.self, on: \CourseSection.$course.$id == \DistrictCourse.$course.$id)
    .filter(DistrictCourse.self, \.$district.$id == district.safeId)
    .all()

The generated query is:

SELECT
    `course_sections`.`id` AS `course_sections_id`,
    `course_sections`.`course_id` AS `course_sections_course_id`,
    `course_sections`.`name` AS `course_sections_name`,
    `course_sections`.`timeDescription` AS `course_sections_timeDescription`,
    `course_sections`.`sourcedId` AS `course_sections_sourcedId`,
    `course_sections`.`createdAt` AS `course_sections_createdAt`,
    `course_sections`.`updatedAt` AS `course_sections_updatedAt`,
    `course_sections`.`deletedAt` AS `course_sections_deletedAt`,
    `district_courses`.`id` AS `district_courses_id`,
    `district_courses`.`district_id` AS `district_courses_district_id`,
    `district_courses`.`course_id` AS `district_courses_course_id`,
    `district_courses`.`createdAt` AS `district_courses_createdAt`,
    `district_courses`.`updatedAt` AS `district_courses_updatedAt`,
    `district_courses`.`deletedAt` AS `district_courses_deletedAt`
FROM
    `course_sections`
    INNER JOIN `district_courses` ON `course_sections`.`course_id` = `district_courses`.`course_id`
WHERE
    `district_courses`.`district_id` = ?
    AND(`course_sections`.`deletedAt` IS NULL
        OR `course_sections`.`deletedAt` > ?)
    AND(`district_courses`.`deletedAt` IS NULL
        OR `district_courses`.`deletedAt` > ?)

This results in a potentially large extraneous amount of data being sent over the wire.

To Reproduce

Run a query with a join + filter

Expected behavior

The query should not return the column set for the joined table unless it's asked for via .field or .with.

Environment

• Vapor Framework version: 4.54.0
• Vapor Toolbox version: N/A
• OS version: macOS 12.0.1

Additional context

N/A

[gwynne]

Some notes:

• This can be mitigated with .fields(for: PrimaryModel.self) or individual .field() invocations
• The default behavior of returning all joined columns can not be changed without breaking compatibility due to the availability of the .all(Model.self, \.$keypath) overload (which would suddenly stop working at runtime without warning).
• This is something we should definitely consider handling differently for Fluent 5, as it's a significant performance consideration.

Since we can't safely change the behavior for existing code, this is a wontfix for Fluent 4.

[jdmcd]

@gwynne makes sense - thank you!