Creating Diesel.rs queries with .and () dynamic number

Question

While playing with Diesel, I was stuck writing a function that takes a vector Stringas input and does the following:

• Combine all Stringwith a big request
• run the query in Connection
• process the result
• return a Vec

If I build the query in one step, it works fine:

fn get_books(authors: Vec<String>, connection: SqliteConnection) {
    use schema::ebook::dsl::*;

    let inner = author
        .like(format!("%{}%", authors[0]))
        .and(author.like(format!("%{}%", authors[1])))
        .and(author.like(format!("%{}%", authors[2])));

    ebook
        .filter(inner)
        .load::<Ebook>(&connection)
        .expect("Error loading ebook");
}

If I try to generate a query in several steps (necessary for working with a variable length of the input vector), I can not compile it:

fn get_books(authors: Vec<String>, connection: SqliteConnection) {
    use schema::ebook::dsl::*;

    let mut inner = author
        .like(format!("%{}%", authors[0]))
        .and(author.like(format!("%{}%", authors[1]))); // <1>

    inner = inner.and(author.like(format!("%{}%", authors[2]))); // <2>

    ebook
        .filter(inner)
        .load::<Ebook>(&connection)
        .expect("Error loading ebook");
}

This causes the following error:

inner = inner.and(author.like(format!("%{}%",authors[2])));
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `diesel::expression::operators::Like`, found struct `diesel::expression::operators::And`

I don’t understand why Rust expects Like, not And. Function a string, marked <1>, returns And, and therefore Andstored in inner.

? , - ? ?

Answer 1

, , complete:

error[E0308]: mismatched types
  --> src/main.rs:23:13
   |
23 |     inner = inner.and(author.like(format!("%{}%", authors[2])));//<2>
   |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected struct `diesel::expression::operators::Like`, found struct `diesel::expression::operators::And`
   |
   = note: expected type `diesel::expression::operators::And<diesel::expression::operators::Like<_, _>, _>`
              found type `diesel::expression::operators::And<diesel::expression::operators::And<diesel::expression::operators::Like<_, _>, diesel::expression::operators::Like<schema::ebook::columns::author, diesel::expression::bound::Bound<diesel::sql_types::Text, std::string::String>>>, _>`

, , . :

expected type `And<Like<_, _>, _>`
   found type `And<And<Like<_, _>, Like<author, Bound<Text, String>>>, _>`

and, , and - and:

fn and<T: AsExpression<Bool>>(self, other: T) -> And<Self, T::Expression>

​​ Diesel SQL . . , Diesel , , Diesel .

, And<_, _> , And<And<_, _>, _>, . , .

, , , - .

-. ( ): BoxableExpression.

, like . , authors . (author = author).

#[macro_use]
extern crate diesel;

use diesel::SqliteConnection;
use diesel::prelude::*;
use diesel::sql_types::Bool;

mod schema {
    table! {
        ebook (id) {
            id -> Int4,
            author -> Text,
        }
    }
}

fn get_books(authors: Vec<String>, connection: SqliteConnection) {
    use schema::ebook::dsl::*;

    let always_true = Box::new(author.eq(author));
    let query: Box<BoxableExpression<schema::ebook::table, _, SqlType = Bool>> = authors
        .into_iter()
        .map(|name| author.like(format!("%{}%", name)))
        .fold(always_true, |query, item| {
            Box::new(query.and(item))
        });

    ebook
        .filter(query)
        .load::<(i32, String)>(&connection)
        .expect("Error loading ebook");
}

fn main() {}

---

, SQL- . , PostgreSQL WHERE col LIKE ANY( subselect ) WHERE col LIKE ALL( subselect ), .