How to build a conditional query

[TheNeikos]

Due to the fact that the filter method returns a new type I'm unable to create a 'dynamic' query. Basically I want to do something like this:

let mut query = submissions.limit(self.settings.limit));

if let Some(user) = self.submitter {
    query = query.filter(user_id.eq(user.id));
}

if let Some(n) = self.title {
    query = query.filter(title.like(n));
}

if let Some(desc) = self.description {
    query = query.filter(description.like(desc));
}

This fails to compile due to the fact that query gets typed right away and it might or might not change.
Is this something that can be addressed? I'd rather not have N! code paths to run through.
A macro would be a suitable replacement, however I am not knowledgeable enough to create it.

[killercup]

One way to solve this is to box the query in each branch (with
.into_boxed()).

Marcel Müller notifications@github.com schrieb am So. 2. Okt. 2016 um
05:24:

Due to the fact that the filter method returns a new type I'm unable to
create a 'dynamic' query. Basically I want to do something like this:

let mut query = submissions.limit(self.settings.limit));

if let Some(user) = self.submitter {
query = query.filter(user_id.eq(user.id));
}

if let Some(n) = self.title {
query = query.filter(title.like(n));
}

if let Some(desc) = self.description {
query = query.filter(description.like(desc));
}

This fails to compile due to the fact that query gets typed right away and
it might or might not change.
Is this something that can be addressed? I'd rather not have N! code
paths to run through.
A macro would be a suitable replacement, however I am not knowledgeable
enough to create it.

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[TheNeikos]

Hm, doing that gives me this error:

error[E0275]: overflow evaluating the requirement `<diesel::query_builder::BoxedSelectStatement<'_, (diesel::types::BigInt, diesel::types::BigInt, diesel::types::Timestamp, diesel::types::Timestamp, diesel::types::BigInt, diesel::types::Text, diesel::types::Text, diesel::types::Nullable<diesel::types::Timestamp>, diesel::types::Integer), models::schema::submissions::table, _> as diesel::boxed_dsl::InternalBoxedDsl<'_, _>>::Output`
   --> src/models/submission.rs:389:55
    |
389 |             query = query.filter(user_id.eq(user.id)).into_boxed();
    |                                                       ^^^^^^^^^^
    |
    = note: consider adding a `#![recursion_limit="128"]` attribute to your crate

Adding that attribute just raises it to 256 :v

---

Edit: Okay, adding into_boxed to the first line makes it compile. So there is that.

Thanks!

[sgrif]

Yeah, you're correct. You'll need to box it before the branch, but you won't need to box it again in each branch. Once it's boxed it stays boxed forever.

[pka]

After an hour or two of trying to compose a query like this, I gave up and looked through all issues finding this solution on page 15 of the closed tickets

This deserves a hint in the documentation (or a more prominent one, in case it's already documented). The ability to compose queries programmatically in a readable way is a fundamental advantage of an ORM compared to raw SQL. Thanks for this great library!

[weiznich]

This deserves a hint in the documentation (or a more prominent one, in case it's already documented)

It is already in the docs. For example here.
Do you have any suggestion where we may add a more prominent hint?

[pka]

What about a sentence in the Getting Started guide like "In cases where you want to conditionally modify a query, use into_boxed to box the pieces of a query into a single type."?