feat: implement IntoSimpleExpr for FunctionCall, ColumnRef, Keyword and CaseStatement

[billy1624]

PR Info

• Closes A FunctionCall cannot be used in .column_as() because it does not implement IntoSimpleExpr #1601

Changes

• Implement IntoSimpleExpr for FunctionCall, ColumnRef, Keyword and CaseStatement

[ikrivosheev]

@billy1624 hello! I think this is not all types... For example:

1. https://docs.rs/sea-query/latest/sea_query/extension/postgres/enum.PgFunction.html
2. https://docs.rs/sea-query/latest/sea_query/extension/postgres/struct.PgFunc.html
3. https://docs.rs/sea-query/latest/sea_query/extension/sqlite/trait.SqliteExpr.html

[billy1624]

Hey @ikrivosheev, I think we shouldn't implement IntoSimpleExpr for the types below.

1. https://docs.rs/sea-query/latest/sea_query/extension/postgres/enum.PgFunction.html (will not be used directly, instead user should construct it via PgFunc)
2. https://docs.rs/sea-query/latest/sea_query/extension/postgres/struct.PgFunc.html (stateless and each method of it returns a FunctionCall which already implemented IntoSimpleExpr
3. https://docs.rs/sea-query/latest/sea_query/extension/sqlite/trait.SqliteExpr.html (this is a trait which extend the functionality of Expression)

[tyt2y3]

This is a rather benign addition, so may be we can add a few unit tests? Just to make sure it compiles.

[ikrivosheev]

Hey @ikrivosheev, I think we shouldn't implement IntoSimpleExpr for the types below.

1. https://docs.rs/sea-query/latest/sea_query/extension/postgres/enum.PgFunction.html (will not be used directly, instead user should construct it via `PgFunc`)

2. https://docs.rs/sea-query/latest/sea_query/extension/postgres/struct.PgFunc.html (stateless and each method of it returns a `FunctionCall` which already implemented `IntoSimpleExpr`

3. https://docs.rs/sea-query/latest/sea_query/extension/sqlite/trait.SqliteExpr.html (this is a trait which extend the functionality of `Expression`)

Thank you for explain!

[billy1624]

This is a rather benign addition, so may be we can add a few unit tests? Just to make sure it compiles.

Just added a few test cases. Please check :)