Remove removed, adds removedAt and removedBy fields to Exercise model

[flacial]

Motivation

In a recent comment, it was addressed that the fields for handling removing an exercise could be improved, and this PR reflects the suggested changes.

Changes

This PR updates the Exercise model to improve how the removed field is handled.

The changes include:

1. Renaming removed to removedAt to match the naming conventions of other date fields in the model.
2. Making removedAt a required field with a default value of null. This ensures that there are only two possible values for the field: null or a date.
3. Adding a new field called removedBy to track the user who removed the exercise.
4. Removing the removed field.

These changes will make it easier to track when an exercise is removed and who removed it, and will improve the overall consistency of the model.

Related issues

• Remove flagged exercises in accordance with design doc #2630

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[codecov]

Codecov Report

Merging #2660 (c81384b) into master (fa4614b) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #2660   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          189       189           
  Lines         3491      3491           
  Branches       966       966           
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  Hits          3491      3491           

Impacted Files	Coverage Δ
__dummy__/getExercisesData.ts	100.00% <ø> (ø)

[flacial]

It's only possible to set removedAt default value to a DateTime, and in the client, we will use it like the following:

if (exercise.removedAt) // do something

If it has a default, value, the above condition will be true, and we'd get an opposite behavior for what we want.

[JasirZaeem]

If you don't set a default value postgresql will set it to null so we don't need to explicitly set the default to null in prisma, postgresql will do it and we'll get the desired behaviour. Being limited to datetime as a default value is due to prisma, prisma doesn't let you set null as the default for an optional value but that is the default behaviour.
Just adding removedAt DateTime? like you have done is enough.

[flacial]

Something I discovered during testing the changes with Apollo playground: removedBy and flaggedBy returns null because they refer to a different model. In order to include them in the result of querying the db for exercises, we'd have to explicitly tell Prisma to include them:

export const exercises = () => {
  return prisma.exercise.findMany({
    include: {
      author: true,
+     flaggedBy: true,
+     removedBy: true,
      module: {
        include: { lesson: true }
      }
    }
  })
}

[JasirZaeem]

Yeah, getting the related fields requires a sql JOIN so prisma will not do the extra work unless needed and requested by us.