Mention calling of SQL functions in Selectable docs

[ISibboI]

In the custom select expression example for the Selectable trait, mention that this can be used to call arbitrary SQL functions as well. Mention the CURRENT_TIMESTAMP function explicitly.

Relates to #3739

[ISibboI]

Seems like some test fails, which is strange, because I didn't change any code. Must be some random failure, because it talks about a deadlock...

[weiznich]

Thanks for this PR. I have some minor wording request otherwise this looks good 👍

[Ten0]

Until we generate the appropriate type from sql_function I don't think that currently works, does it? (#3745)

[ISibboI]

Thanks for the review! I have now added a field to the doctest below this comment that gets initialised with diesel::dsl::now. It seems to complain about some trait not being implemented:

error[E0277]: the trait bound `expression::select_by::SelectBy<UserPost, _>: load_dsl::private::CompatibleType<_, _>` is not satisfied
    --> src/expression/mod.rs:596:12
     |
37   |     .first(connection)?;
     |      ----- ^^^^^^^^^^ the trait `load_dsl::private::CompatibleType<_, _>` is not implemented for `expression::select_by::SelectBy<UserPost, _>`
     |      |
     |      required by a bound introduced by this call
     |
     = help: the trait `load_dsl::private::CompatibleType<U, DB>` is implemented for `expression::select_by::SelectBy<U, DB>`
     = note: required for `SelectStatement<FromClause<JoinOn<Join<table, table, Inner>, Grouped<Eq<Nullable<user_id>, Nullable<id>>>>>, ..., ..., ..., ..., ...>` to implement `LoadQuery<'_, _, _>`
     = note: the full type name has been written to '/tmp/rustdoctestBQRQaQ/rust_out.long-type-2905737123615719141.txt'
note: required by a bound in `first`
    --> /home/sibbo/git/diesel/diesel/src/query_dsl/mod.rs:1780:22
     |
1780 |         Limit<Self>: LoadQuery<'query, Conn, U>,
     |                      ^^^^^^^^^^^^^^^^^^^^^^^^^^ required by this bound in `RunQueryDsl::first`

Is this what you mean?

[Ten0]

Actually for the particular case of dsl::now given how it's defined it would work, however if a user were to define a custom function that does CURRENT_TIMESTAMP() using sql_function! that wouldn't work.

I suspect your error comes from the type you are selecting to, and you may get a more explicit error message using check_for_backend.

[ISibboI]

I understand, thanks. Then I should update the documentation.