How to perform joins on non-primary columns and complex select statements in loopback 4?

Describe the bug

Hi i am new to loopback and need to create an API with business logic that contains several complex queries. I cant seem to find a way to join two tables with non-primary columns. For example, select * from e_c left join e on e.ed=e_c.ed Table relations: e_c.ed fk to ed table on id e.ed fk to ed table on id

On the same note writing complex select statements are also needed. For example, select (case when completed_on is null then 1 else 0 end) flag from e or select sum(rating) from f_b

Kindly provide some method by which these can be achieved via loopback 4

Logs

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Additional information

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Reproduction

ソースファイル、テスト、エントリポイントのいずれも指定されていません。まず、非プライマリ列を使った join と複雑な SELECT の例を参照しながら、LoopBack 4 の relation API と query API を確認してください。文書化されたサポート対象のアプローチを特定するか、制限事項と必要なスコープを明確に文書化できれば完了です。