feat(sql): add callback signature to expr() with alias parameter

```ts
const qb1 = orm.em.createQueryBuilder(Book2);
qb1.select('*').where({
  // using the alias of joined author via expr()
  author: { [expr(alias => `lower(${alias}.name)`)]: '...' } },
});
```

Closes #2405

packages/core/src/utils/QueryHelper.ts

@@ -253,8 +253,13 @@ export class QueryHelper {
 /**
  * Helper for escaping string types, e.g. `keyof T -> string`.
  * We can also pass array of strings to allow tuple comparison in SQL drivers.
+ * Another alternative is to use callback signature, which will give us the current alias in its parameter.
  */
-export function expr<T = unknown>(sql: (keyof T & string) | (keyof T & string)[]): string {
+export function expr<T = unknown>(sql: (keyof T & string) | (keyof T & string)[] | ((alias: string) => string)): string {
+  if (sql instanceof Function) {
+    return sql('[::alias::]');
+  }
+
   if (Array.isArray(sql)) {
     return Utils.getPrimaryKeyHash(sql);
   }

packages/knex/src/query/ObjectCriteriaNode.ts

@@ -36,7 +36,7 @@ export class ObjectCriteriaNode extends CriteriaNode {
       } else if (childNode.shouldRename(payload)) {
         o[childNode.renameFieldToPK(qb)] = payload;
       } else if (primaryKey || virtual || operator || customExpression || field.includes('.') || ![QueryType.SELECT, QueryType.COUNT].includes(qb.type ?? QueryType.SELECT)) {
-        o[field] = payload;
+        o[field.replace(/\[::alias::]/g, alias!)] = payload;
       } else {
         o[`${alias}.${field}`] = payload;
       }

tests/QueryBuilder.test.ts

@@ -1,5 +1,5 @@
 import { inspect } from 'util';
-import { LockMode, MikroORM, QueryFlag, QueryOrder } from '@mikro-orm/core';
+import { expr, LockMode, MikroORM, QueryFlag, QueryOrder } from '@mikro-orm/core';
 import type { PostgreSqlDriver } from '@mikro-orm/postgresql';
 import { CriteriaNode } from '@mikro-orm/knex';
 import { MySqlDriver } from '@mikro-orm/mysql';
@@ -420,7 +420,7 @@ describe('QueryBuilder', () => {
     expect(qb1.getParams()).toEqual(['{"foo":"bar"}']);
 
     const qb2 = orm.em.createQueryBuilder(Book2);
-    qb2.select('*').where({ 'json_contains(`e0`.`meta`, ?) = ?': [{ foo: 'baz' }, false] });
+    qb2.select('*').where({ [expr(a => `json_contains(\`${a}\`.\`meta\`, ?) = ?`)]: [{ foo: 'baz' }, false] });
     expect(qb2.getQuery()).toEqual('select `e0`.*, `e0`.price * 1.19 as `price_taxed` from `book2` as `e0` where json_contains(`e0`.`meta`, ?) = ?');
     expect(qb2.getParams()).toEqual(['{"foo":"baz"}', false]);
   });
@@ -1644,6 +1644,20 @@ describe('QueryBuilder', () => {
       'where (`e1`.`name` = ? or not (`e1`.`name` = ?))');
   });
 
+  test('select with auto-joining and alias replacement via expr()', async () => {
+    const qb1 = orm.em.createQueryBuilder(Book2, 'a');
+    qb1.select('*').where({
+      $or: [
+        { author: { name: 'test' } },
+        { author: { [expr(a => `lower(${a}.name)`)]: 'wut' } },
+      ],
+    });
+    expect(qb1.getQuery()).toEqual('select `a`.*, `a`.price * 1.19 as `price_taxed` ' +
+      'from `book2` as `a` ' +
+      'left join `author2` as `e1` on `a`.`author_id` = `e1`.`id` ' +
+      'where (`e1`.`name` = ? or lower(e1.name) = ?)');
+  });
+
   test('select by PK via operator', async () => {
     const qb1 = orm.em.createQueryBuilder(Author2, 'a');
     qb1.select('*').where({ $in: [1, 2] });