Trait `diesel::expression::NonAggregate` is not implemented for `diesel::dsl::CountStar`

[ondj]

Setup

Versions

• Rust: 1.27.0
• Diesel: 1.3.2
• Database: PostgreSQL
• Operating System: Windows 10

Feature Flags

diesel = { version = "1.3.0", features = ["postgres", "chrono", "r2d2"] }

Problem Description

I am trying to get results from db grouped by column serial_to and get serial_to with number of entries for that serial.

What are you trying to accomplish?

Send to database SQL query like this:

SELECT COUNT(*), serial_to FROM inet GROUP BY serial_to

What is the expected output?

Vec<(i32, i64)>

What is the actual output?

Compilation error:

error[E0277]: the trait bound `diesel::dsl::CountStar: diesel::expression::NonAggregate` is not satisfied
  --> src/stats.rs:25:10
   |
25 |         .select((inet::serial_to, count_star()))
   |          ^^^^^^ the trait `diesel::expression::NonAggregate` is not implemented for `diesel::dsl::CountStar`
   |
   = note: required because of the requirements on the impl of `diesel::Expression` for `(database_schema::inet::columns::serial_to, diesel::dsl::CountStar)`
   = note: required because of the requirements on the impl of `diesel::query_dsl::select_dsl::SelectDsl<(database_schema::inet::columns::serial_to, diesel::dsl::CountStar)>` for `database_schema::inet::table`

error[E0599]: no method named `group_by` found for type `diesel::query_builder::SelectStatement<database_schema::inet::table, diesel::query_builder::select_clause::SelectClause<(database_schema::inet::columns::serial_to, diesel::dsl::CountStar)>>` in the current scope
  --> src/stats.rs:26:10
   |
26 |         .group_by(inet::serial_to)
   |          ^^^^^^^^ private field, not a method
   |
   = note: the method `group_by` exists but the following trait bounds were not satisfied:
           `&diesel::query_builder::SelectStatement<database_schema::inet::table, diesel::query_builder::select_clause::SelectClause<(database_schema::inet::columns::serial_to, diesel::dsl::CountStar)>> : diesel::GroupByDsl<_>`
           `&mut diesel::query_builder::SelectStatement<database_schema::inet::table, diesel::query_builder::select_clause::SelectClause<(database_schema::inet::columns::serial_to, diesel::dsl::CountStar)>> : diesel::GroupByDsl<_>`

Are you seeing any additional errors?

No

Steps to reproduce

Database schema can be simple:

table! {
    inet (hash) {
        hash -> Binary,
        serial_to -> Int4,
    }
}

This is compiled correctly and works:

let stats: Vec<i32> = inet::table
        .select(inet::serial_to)
        .group_by(inet::serial_to)
        .load(&connection)?;

But this shows complication errors:

let stats: Vec<(i32, i64)> = inet::table
        .select((inet::serial_to, count_star()))
        .group_by(inet::serial_to)
        .load(&connection)?;

Checklist

• I have already looked over the issue tracker for similar issues.
• This issue can be reproduced on Rust's stable channel. (Your issue will be
  closed if this is not the case)

[vmalloc]

Also hitting this issue and was wondering if there is a workaround to getting it work...

[vmalloc]

I managed to work around it by replacing count_star() with diesel::dsl::sql<diesel::sql_types::BigInt>("count(*)"). Yuck, but works...

[sgrif]

Diesel doesn't currently support .group_by, and for that reason we don't allow mixing aggregate/non-aggregate expressions. You can use the workaround above until we introduce support for group by