How to use PreloadWhere?

[absolutejam]

I've discovered PreloadWhere and want to use it to control some of my queries so that they only select data conditional depending upon another table (ie. filter on inner join)... but I can't for the life of me figure out what to do with this mod!

I know the signature is func(from, to string) []bob.Expression, but I can't seem to satisfy bob.Expression. I've tried some partial queries, just some clauses, and everything in-between 🤦

Any examples would be much appreciated!

[stephenafamo]

PreloadWhere is a mod that is passed to a preload.

For example, if we only wanted to preload a user's group if the group is 10, then we do this:

models.Users(
  models.Preload.User.Group(
    psql.PreloadWhere(func(from, to string) {
      return []bob.Expression{
        models.Groups.Columns.AliasedAs(to).ID.EQ(psql.Raw(10)),
      },
    }),
  ),
)

NOTE: This will not prevent loading users in other groups. All users will still be loaded.

The reason the function has the signature func (from, to string) []bob.Expression is because when doing preloads, the table name is usually suffixed with a random string of numbers, so the from and to allow you to know what the table is aliased as.

[absolutejam]

Thanks for clarifying - I will give that a go a bit later.

NOTE: This will not prevent loading users in other groups. All users will still be loaded.

Interesting, thanks. So, is there currently no way to change the join into an INNER JOIN, to restrict the initial query?

[stephenafamo]

No, but if you do another join on the same table, the query planner should be able to understand it all