Can't query Json field

[alper]

Hi,

I have models.rs like this:

use diesel::sql_types::Json;

#[derive(Queryable)]
pub struct GMapsLocation {
    pub id: i32,
    pub place_id: String,
    pub data: Json,
}

And trying to do this:

    let results: Vec<GMapsLocation> = gmaps_locations
        .select((id, place_id, data))
        .load(&connection)
        .expect("Erorr loading locations");

And then I get this:

src/main.rs:18:10
     |
18   |         .load::<GMapsLocation>(&connection)
     |          ^^^^ the trait `Queryable<diesel::sql_types::Json, _>` is not implemented for `diesel::sql_types::Json`
     |
     = note: required because of the requirements on the impl of `Queryable<(Integer, Text, diesel::sql_types::Json), _>` for `(i32, std::string::String, diesel::sql_types::Json)`
     = note: 1 redundant requirement hidden
     = note: required because of the requirements on the impl of `Queryable<(Integer, Text, diesel::sql_types::Json), _>` for `foursquare_mapper::models::GMapsLocation`
     = note: required because of the requirements on the impl of `LoadQuery<_, foursquare_mapper::models::GMapsLocation>` for `SelectStatement<table, query_builder::select_clause::SelectClause<(foursquare_mapper::schema::gmaps_locations::id, foursquare_mapper::schema::gmaps_locations::place_id, foursquare_mapper::schema::gmaps_locations::data)>>`
note: required by a bound in `load`

No idea what is happening and I can't find an example of how to query Json anywhere in the docs. This also shows how to insert data: https://docs.diesel.rs/diesel/pg/types/sql_types/struct.Jsonb.html

[alper]

Inserts are working with this setup though as per the example on the Jsonb page.

[alper]

OK. I found out what the issue was.

Just like the input value from the documentation is a serde_json::Value, the return value from the query is also a serde_json::Value. I have no clue what diesel::sql_types::Json is for then.

Also, we don't get the struct back that's in models but a vector of tuples instead. (Why have a model?)

So the working code is:

let results = gmaps_locations
    .select((id, place_id, data))
    .load::<(i32, String, serde_json::Value)>(&connection)
    .expect("Erorr loading locations");

[weiznich]

There is no need to use a tuple here. Any struct that implements Queryable the same way will work there. That includes structs that use a serde_json::Value field at the corresponding location.
As for the use of types from diesel::sql_types: Checkout the API documentation of that module.

[alper]

But why didn't it work with my original struct? The one that defines the table?

[weiznich]

I cannot see how your struct defines a table. That's what the table! macro is doing. As general rule: Never use a type from diesel::sql_types to hold any value. They are zero sized types so they cannot hold values. If you use the same type in your struct as in your tuple it will work.

[alper]

And the Queryable should not have a Json in its definition? Sorry, I've gone through the documentation as best I could.

[alper]

OK. I changed my model to:

pub struct GMapsLocation {
    pub id: i32,
    pub place_id: String,
    pub data: serde_json::Value,
}

And now it's working without the tuple.