Custom scalars support

[dmytro-shpak]

Hi!
The mst-gql looks very promising tool for me. But i have not found the Custom Scalars support. That feature could be implemented by --fieldOverrides flag or by manual extending of Model.base.

I am working on --customScalars feature now (#370). The root idea is to allow to inject custom scalars implemented with types.custom to generated code by scalar name.

Is it a good idea or custom scalars could be handled in some other common way?
Is it a good idea to add that feature to the main mst-gql project?

[dmytro-shpak]

It looks like more correct idea is to extend the ** --fieldOverrides** by one more field to allow import custom scalars from some file

[dmytro-shpak]

The version with extended --fieldOverrides is created #375