Add `exists` to expression DSL functions

[jimmycuadra]

This will allow the user to check for the existence of a record without loading it.

[shssoichiro]

I'd like to work on this, if no one else has started it.

[killercup]

Sure, go ahead. I don't think anyone is working on this. Have you seen
any? The implementation might be similar.

Josh Holmer notifications@github.com schrieb am Do. 15. Sep. 2016 um
13:43:

I'd like to work on this, if no one else has started it.

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[shssoichiro]

I'm struggling a bit getting my initial test to pass. I have an implementation which compiles, which is based off of how count works, but my ExistsDsl trait doesn't seem to be implemented for the correct traits. I'm getting:

---- query_dsl::exists_dsl::ExistsDsl_0 stdout ----
    error: no method named `exists` found for type `users::table` in the current scope
  --> <anon>:15:20
   |
15 | let exists = users.exists().first(&connection);
   |                    ^^^^^^
   |
   = note: the method `exists` exists but the following trait bounds were not satisfied: `diesel::query_builder::SelectStatement<(diesel::types::Integer, diesel::types::Text), (users::columns::id, users::columns::name), users::table> : diesel::exists_dsl::ExistsDsl`, `&users::table : diesel::QuerySource`, `&users::table : diesel::query_builder::AsQuery`, `_ : diesel::exists_dsl::ExistsDsl`, `_ : diesel::Expression`, `&users::table : diesel::query_builder::Query`, `&mut users::table : diesel::QuerySource`, `&mut users::table : diesel::query_builder::AsQuery`, `_ : diesel::exists_dsl::ExistsDsl`, `_ : diesel::Expression`, `&mut users::table : diesel::query_builder::Query`
   = help: items from traits can only be used if the trait is implemented and in scope; the following trait defines an item `exists`, perhaps you need to implement it:
   = help: candidate #1: `diesel::exists_dsl::ExistsDsl`

When currently my ExistsDsl implementation looks like this:

pub trait ExistsDsl: AsQuery {
    type Output;

    fn exists(self) -> Self::Output;
}

impl<T, ST> ExistsDsl for T
    where T: QuerySource + AsQuery<SqlType=ST>,
          T::Query: ExistsDsl + Expression {
    type Output = SelectStatement<Bool, Exists<T::Query>, ()>;

    fn exists(self) -> Self::Output {
        select(exists(self.as_query()))
    }
}