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Fix merging of Input objects and enum types #1672

Merged
merged 2 commits into from
Apr 5, 2022
Merged

Fix merging of Input objects and enum types #1672

merged 2 commits into from
Apr 5, 2022

Commits on Apr 5, 2022

  1. Fix merging of Input objects and enum types 

    At a high level, the merging guiding principle is meant to be:
- output types are merged by "union": meaning an element of an output
  types can (in general) be in the supergraph as soon as it is in any
  subgraph. The idea being that it's ok (and desirable) that different
  subgraph can return different "sub-parts" of an output type and the
  query planner is here direct queries appropriately. Of course, in
  practice, the "composition validation" algorithm imposes some limits
  to this based on the reachability of the type.
- input types are merged by "intersection": meaning that an element
  an input type is only in the supergraph if it is in all the subgraph
  defining the type in question. The idea being that the gateway don't
  control the inputs it gets, and so in general, all subgraphs needs
  be able to handle all the inputs exposed by the supergraph API.

This guiding principle was not implemented properly for both Input
Object types and Enum types and this is fixed by this commit.

Note that:
- for input object types, this has the consequence of aligning the
  merging of input fields with that of (output) field arguments. Indeed,
  passing multiple arguments is extremely similar semantically to
  passing a single input object having the same fields, so it does not
  make sense for the behiavour of those to differ, and it doesn't
  anymore after this patch.
- enum types are a special case because they can be use as both output
  types and input types (this is true for scalar too, but scalar merging
  is trivial). The patch makes the behaviour depend on how the enum
  type is used:
  - if it is only ever used as an input type, then we merge it by
    "intersection".
  - if it is only ever used as an output type, then we merge it by
    "union".
  - otherwise, if it used as both, then merging is essentially
    "equality", meaning that we have a composition error unless all
    the subgraphs defined the same values.
    Sylvain Lebresne committed Apr 5, 2022
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  2. Update changelog and address review comments

    Sylvain Lebresne committed Apr 5, 2022
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