feat(tags): allow smart tags to indicate a column has a default

[benjie]

Description

PostGraphile automatically determines when a table column has a default, and thus makes that field nullable on the input to a create mutation. However, when we are looking at a different source (for example a view) we cannot determine whether the column has a default or not. If you use the smart tag @primaryKey then we'll know that that column is not-null, but it does not indicate that it has a default, and so we think it's required. Often this is not the case (many primary keys have defaults), so in this PR we introduce the @hasDefault smart tag that you can use to indicate that a column (e.g. your @primaryKey column) has a default and thus should be exposed as nullable.

Fixes graphile/crystal#1544

Performance impact

Negligible.

Security impact

None known.

[bfelbo]

@benjie Thanks a lot for adding this 🙌

This doesn't seem to work though if the column marked with @hasDefault also has an @omit create.

For instance, we have a NOT NULL ranking_score column that should be set by a PostGraphile plugin (never provided by a user, thereby the @omit create). However, marking the column @hasDefault + @omit create and setting the value using a plugin as shown below still throws null value in column "ranking_score" violates not-null constraint.

import { makeWrapResolversPlugin } from "graphile-utils"

export default makeWrapResolversPlugin({
  Mutation: {
    createGameComment: async(resolver, source, args, context, info) => {
      args.input.gameComment.rankingScore = 1.0
      const result = await resolver(source, args, context, info)
      return result
    }
  }
})

EDIT: Clarified the importance of @omit create.

[bfelbo]

The ideal solution would be that adding @hasDefault not only makes the column nullable, but also allows a plugin to insert a value for that column despite of @omit create.

I can create a new issue for this if you'd prefer.

[benjie]

Please do.