fix(typeMerging): fix interface merging

[yaacovCR]

thanks go to @gmac for finding this bug and contributing test code
see: #1911 (comment)

[yaacovCR]

Note that mergeTypes: true has to be explicitly added. mergeTypes is false by default, but any type with merge configuration is automatically merged. This does not occur automatically for the interface, and so mergeTypes has to be explicitly set to true.

[theguild-bot]

The latest changes of this PR are available as alpha in npm: 6.0.19-alpha-b8551da1.0

Quickly update your package.json by running:

npx match-version @graphql-tools 6.0.19-alpha-b8551da1.0

[gmac]

Dang, while I don't have any specific use for this kind of interface merging, I sure am glad I can! I'll write up another section in the type merging guide on combined interface strategies. This is a woefully under-discussed concern across GraphQL projects for something that really isn't an obscure edge case.

[yaacovCR]

Huh, my quick survey of how this works in federation is that interfaces seem to be required to be identical across subschemas. I did not realize that.

A few links:
https://spectrum.chat/apollo/apollo-federation/how-to-use-interfaces-across-services~5132179a-545d-49a4-b50f-67ae9975b4e4
https://spectrum.chat/apollo/apollo-federation/how-to-use-interfaces-across-services~5132179a-545d-49a4-b50f-67ae9975b4e4
https://github.com/apollographql/apollo-server/issues/2849
https://github.com/apollographql/apollo-server/issues/4361

[yaacovCR]

Just a separate note relating back to mergeTypes,

graphql-tools/packages/stitch/tests/extendedInterface.test.ts

Lines 34 to 36 in b8551da

	extend interface Slot {
	name: String!
	}

Gateway type definition, if mergeTypes set to true, could use type instead of extend.

Not sure yet which one I would prefer based on style, I think I would prefer extend and I think it doesn't really matter, but I guess important to mention that type merging if enabled will also make gateway types no longer replace subschema types, but will also be merged.

[yaacovCR]

Thanks again @gmac for the huge amount of work you are putting in to type merging!!!

[gmac]

All praise goes to you, friend. It’s an honor to provide backup on such a great project.

The links you post pretty well summarize the state of cross-service interfaces in my research... inconclusive and full of fairly bad options.