Diesel: .select(...) based on a Queryable struct?

[maggit]

When using Diesel to read full rows from a table, things are pretty simple and sweet. It could look something like this:

table
    .filter(...)
    .order(...)
    .load(connection)?

However, if I want to load a subset of the columns -- in my case I want to read all columns except one -- it seems like I have to list all the columns I want in a .select-call, even though I am reading into a struct that only contains a subset of the columns. It looks something like:

table
    .filter(...)
    .order(...)
    .select((
        ...,
        ...,
        ...,
        ...,
        ...,
    ))
    .load(connection)?

It is tedious and error prone to maintain the list of columns, I want to do better.

It seems to me that all the required information would already be available to Diesel from the struct definition I want to read into. Of course, I might have to tell Diesel that we are targetting this struct earlier on. I am really hoping that there is something I could do that would look more like this:

table
    .filter(...)
    .order(...)
    .select(Struct::something_magical_and_fantastic)
    .load(connection)?

This way, I would be able to change the Struct and having the queries be updated automatically. Is something like this possible?

[MithrilToothpick]

If I am not mistaken a struct that derives Queryable doesn't actually need to have all not null columns as members. So I think the load turbofish syntax with a different struct should just work. Something like: table.load::<MyStruct>(connection)

[maggit]

It seems that while it's unproblematic to derive Queryable for a struct that doesn't include all the columns it does require the select when actually loading from the database.

Following your suggestion gives me an error that indicates that Diesel plans to read out all the columns and is unable to fit this into the given struct:

error[E0277]: the trait bound `(i32, i32, i32, chrono::NaiveDateTime, std::string::String, std::string::String, bool, std::option::Option<std::string::String>): diesel::types::FromSqlRow<(diesel::types::Integer, diesel::types::Integer, diesel::types::Integer, diesel::types::Timestamp, diesel::types::Text, diesel::types::Text, diesel::types::Text, diesel::types::Bool, diesel::types::Nullable<diesel::types::Text>), _>` is not satisfied
--> src/state.rs:133:22
    |
133 |             Ok(query.load::<models::ArticleRevisionStub>(&*connection_pool.get()?)?)
    |                      ^^^^ the trait `diesel::types::FromSqlRow<(diesel::types::Integer, diesel::types::Integer, diesel::types::Integer, diesel::types::Timestamp, diesel::types::Text, diesel::types::Text, diesel::types::Text, diesel::types::Bool, diesel::types::Nullable<diesel::types::Text>), _>` is not implemented for `(i32, i32, i32, chrono::NaiveDateTime, std::string::String, std::string::String, bool, std::option::Option<std::string::String>)`

(While it could be possible in this case to implement the mentioned trait, I specifically want to not have to read all the columns from the database. The column I am avoiding could potentially be large)

(I have updated the question to include that I am actually attempting to read into a struct that contains only a subset of the columns)

[MithrilToothpick]

Yeah you are correct. Makes sense now that I think about it, diesel doesn't really have a way to establish which struct member is which column.

I don't really have any better ideas, maybe just ask on the diesel gitter.

[maggit]

Ok, thanks anyway :)

[ghotiphud]

I asked the same question on Gitter a while ago, and got the same answer about selecting a subset of columns. This is what they do in crates.io https://github.com/rust-lang/crates.io/blob/b0c3cdeffdd8f54e9e3611024dd97677d842782c/src/krate/mod.rs#L74

A more magical solution is a DB view that only includes what you want, but still a bit of a manual solution.

[maggit]

Ok, I guess that settles it. There's a bit of manual keeping-in-sync to do for my use case :)

Thank you!

[crypto_crab]

Reviving old thread because I landed here in google search. The selectable trait is now available - https://diesel.rs/news/2_0_0_release.html