Allow for generic functions that take BoxedSelectStatement as an arg #2936
Replies: 5 comments 1 reply
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Your "workaround" is basically the correct solution. Let me first explain why this happens: As an additional note: The |
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Thanks for the help and advice. I've switched out The function is now defined as: (I've also tried using It's probably something simple I'm missing, so any pointers would be much appreciated. Regarding the public/private diesel API. As far as possible I'd like to rely only on public APIs that are going to continue to be available (though practically, as long as there is anything that provides a way to "conditionally modify a query, but need the type to remain the same", it will be easy enough to change need be). I'm now relying on I'll try to put together a minimal example to reproduce the unnamed type bound issue too. Thanks again for your help. |
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This likely indicates the following missing bounds for your function: T: Table + AsQuery,
T::Query: BoxedDsl<'static, Pg>(This is again one of the not so great rustc error messages. I think I've already reported that one and it seems like that is really hard to fix.)
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Well that was quite an adventure! Sorry it's taken me a while to reply — it's taken me quite a while to get it working, but it's not pretty: I've tried adding various There could well be another trait that would dramatically simplify the above, but I spent some time searching and couldn't find one. Are there other traits like It really feels like there must be a better way. |
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I've added a step to step description on how to solve such issues in general below, see there for a simpler solution.
It really depends on what you are looking for here. Yes there are trait bounds that simplify the bounds a bit, but that requires to read and understand the compiler error messages completely. Basically that's the price you need to pay for checking things in the type system + rustc trying to be to helpful. There is really not much we can do here to improve the error messages, at least not without major changes to the compiler. I would be happy if someone drives those changes, but I personally won't do that in my free time.
There is definitively no trait impl missing for |
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I will use this post to write down a step by step description how I normally solve such generic issues. In the result this will lead to a much simpler solution. First let us use a "simplified" version of the OP's example: fn check_exists<'a, T>(
conn: &PgConnection,
check: dsl::IntoBoxed<'a, T, Pg>,
) -> Result<(), diesel::result::Error>
{
use diesel::dsl::{exists, select};
select(exists(check))
.get_result::<bool>(&conn)?
.then(|| {})
.expect("Does not exist");
Ok(())
}This changes a few minor bits, so that it is easily possible to test this function outside of the project of the OP. In detail, it changes the error type, it changes one line of error handling to a call to Now if we compile this version rustc emits the following error message: This is one of the worser diesel error messages, but unfortunately nothing we can easily fix now. That written it mentions the bound fn check_exists<'a, T>(
conn: &PgConnection,
check: dsl::IntoBoxed<'a, T, Pg>,
) -> Result<(), diesel::result::Error>
+ where
+ T: BoxedDsl<'a, Pg>,
{
use diesel::dsl::{exists, select};
select(exists(check))
.get_result::<bool>(&conn)?
.then(|| {})
.expect("Does not exist");
Ok(())
}
This leads to the following compiler error message: That's quite a lot of text, so let's try to split it down a bit. First of all, the overflow message it gone, so it seems like the previous change did help. That's good 🎉 Now let's go over the error messages one by one: This one says that it is missing a certain trait bound ( fn check_exists<'a, T>(
conn: &PgConnection,
check: dsl::IntoBoxed<'a, T, Pg>,
) -> Result<(), diesel::result::Error>
where
T: BoxedDsl<'a, Pg>,
+ dsl::IntoBoxed<'a, T, Pg>: SelectQuery,
{
use diesel::dsl::{exists, select};
select(exists(check))
.get_result::<bool>(conn)?
.then(|| {})
.expect("Does not exist");
Ok(())
}Ok, one error message fixed, 7 error message remaining. Let's look at the next one: This is basically like the last one, but with a different trait. The compiler suggests to add fn check_exists<'a, T>(
conn: &PgConnection,
check: dsl::IntoBoxed<'a, T, Pg>,
) -> Result<(), diesel::result::Error>
where
T: BoxedDsl<'a, Pg>,
- dsl::IntoBoxed<'a, T, Pg>: SelectQuery,
+ dsl::IntoBoxed<'a, T, Pg>: SelectQuery + diesel::expression::subselect::ValidSubselect<()>,
{
use diesel::dsl::{exists, select};
select(exists(check))
.get_result::<bool>(conn)?
.then(|| {})
.expect("Does not exist");
Ok(())
}
That fixes another error message. Let's have a look at the next one: This one complains about a missing This can be understand as if we add the bound fn check_exists<'a, T>(
conn: &PgConnection,
check: dsl::IntoBoxed<'a, T, Pg>,
) -> Result<(), diesel::result::Error>
where
T: BoxedDsl<'a, Pg>,
dsl::IntoBoxed<'a, T, Pg>: SelectQuery + ValidSubselect<()>,
+ dsl::Select<SelectStatement<()>, Exists<dsl::IntoBoxed<'a, T, Pg>>>: LoadQuery<PgConnection, bool>,
{
use diesel::dsl::{exists, select};
select(exists(check))
.get_result::<bool>(&conn)?
.then(|| {})
.expect("Does not exist");
Ok(())
}
This fixes another error message. This leaves 4 error message. The next two error messages are: Note that:
That means we can just ignore them, as they are already fixed 🎉 Let's look at the next error message: Again this one contains the same Leaves one error message: That one is a bit strange. At this point we already changed quite a lot of details, by adding 3 additional trait bounds. That's the point where I normally just check and see which messages are fixed now and which are not. It turns the code compiles now, so it seems like the error message is already fixed by one of the former fixes 🎉 This leaves us with the following finial definition of your fn check_exists<'a, T>(
conn: &PgConnection,
check: dsl::IntoBoxed<'a, T, Pg>,
) -> Result<(), diesel::result::Error>
where
T: BoxedDsl<'a, Pg>,
dsl::IntoBoxed<'a, T, Pg>: SelectQuery + ValidSubselect<()>,
dsl::Select<SelectStatement<()>, Exists<dsl::IntoBoxed<'a, T, Pg>>>: LoadQuery<PgConnection, bool>,
{
use diesel::dsl::{exists, select};
select(exists(check))
.get_result::<bool>(&conn)?
.then(|| {})
.expect("Does not exist");
Ok(())
} |
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Firstly, it's entirely possible I'm just doing it wrong, so if there's a better way, please let me know.
I'm trying to create some helper functions to dry up my code a bit. In a number of places, I have access to a
BoxedSelectStatement, and I'd like to be able to do something with it, for instance, check whether it exists. Something like the following:where for convenience:
However, I receive the following compile error:
I'm not exactly sure what this is telling me, but to some extent I would have expected that a
BoxedSelectStatementwould always work inside aselect(exists()). Nonetheless, I can try adding a constraint to work around the problem as follows:And that works, except
SelectClauseisn't public, so I'm testing the above approach using a fork of diesel where I've addedpub use self::select_clause::SelectClause;todiesel/src/query_builder/mod.rs.Am I doing it wrong (i.e., is there something other than a
BoxedSelectStatementI should be using for this use-case)?And if I'm not doing it wrong, should diesel provide trait implementation such that
QueryFragment<_>for aBoxedSelectStatementis guaranteed? Or is the approach of adding the constraint in the function the right way to go, and presumablySelectClauseshould be made public?Beta Was this translation helpful? Give feedback.
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