Using generic types as query arguments is not possible

[manadart]

Using a generic struct in queries is not possible

Consider preparing this query:

INSERT INTO unit_state_relation(*) VALUES ($unitStateKeyVal.*)

And passing a materialised unitStateKeyVal[int] to the call. This results:

preparing relation state insert query: preparing:: cannot prepare statement: input expression: parameter with type "unitStateKeyVal" missing (have "unitStateKeyVal[int]"): (*) VALUES ($unitStateKeyVal.*)

Trying to include the type parameter does not help. Preparing this:

INSERT INTO unit_state_relation(*) VALUES ($unitStateKeyVal[int].*)

Results in:

preparing relation state insert query: preparing:: cannot parse expression: column 44: unqualified type, expected unitStateKeyVal.* or unitStateKeyVal.<db tag> or unitStateKeyVal[:]

[Aflynn50]

Just had a little dig into this. It seems very possible but there are a few tripups.

The simple and by far the most used case is very easy. If you have a generic type instantiated with a single scalar type argument then then:

reflect.TypeOf(Generic[int]{}).Name() == "Generic[int]"

This is fine.

It starts getting complicated when you have user defined types in there, or multiple types, for instance

reflect.TypeOf(Generic[int, string]{}).Name() == "Generic[int,string]"
reflect.TypeOf(Generic[MyString]{}).Name() == "Generic[main.MyString]"

In the first case the space is omitted and in the second case the package name is included in the type argument name. Whats more, the reflect package provides no way to get more information about these type arguments.

This is of course surmountable, but currently SQLair looks for the types names in the query in the types passed to prepare by looking up their names in a map. This would not longer work since we couldn't expect the user to exactly match the right format in these edge cases.

Of course we could just force the users to match the exact names in these cases, but if we did it it would be good to do it properly.

[Aflynn50]

As a workaround you can do:

type GenericInt Generic[int]