Abstract filter argument

rfcs/AbstractFilter/AbstractFilterArgumentSpec.md

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+# Abstract Filter Argument
+
+Status: Strawman
+
+## Directive
+
+```graphql
+directive @limitTypes on FIELD_DEFINITION
+```
+
+### Examples
+
+```graphql
+type Query {
+  allPets(first: Int, only: [String] @limitTypes): [Pet]
+}
+
+interface Pet {
+  name: String!
+}
+```
+
+```graphql
+type Query {
+  allPetsConnection(
+    first: Int
+    after: String
+    only: [String] @limitTypes
+  ): PetConnection
+}
+
+# TODO: connection types
+
+interface Pet {
+  name: String!
+}
+```
+
+## Schema Validation
+
+The `@limitTypes` directive must not appear on more than one argument on the
+same field.
+
+The `@limitTypes` directive may only appear on an argument that accepts a
+(possibly non-nullable) list of (possibly non-nullable) String.
+
+The `@limitTypes` directive may only appear on an argument to a field whose
+named return type is a connection type (that is to say, conforming to
+[the GraphQL Cursor Connections Specification's Connection Type](https://relay.dev/graphql/connections.htm#sec-Connection-Types))
+over an abstract type, or to a field whose return type is a list and the named
+return type is an abstract type.
+
+## Execution
+
+The `@limitTypes` directive places requirements on the {resolver} used to
+satisfy the field. Implementers of this specification must honour these
+requirements.
+
+### Coerce Allowed Types
+
+The input to the filter argument is a list of strings, however this must be made
+meaningful to the resolver such that it may perform its filtering - thus we must
+resolve it into a list of valid concrete object types that are possible in this
+position.
+
+CoerceAllowedTypes(abstractType, typeNames):
+
+- Let {possibleTypes} be a set of the possible types of {abstractType}.
+- Let {allowedTypes} be an empty unordered set of object types.
+- For each {typeName} in {typeNames}:
+  - Let {type} be the type in the schema named {typeName}.
+  - If {type} does not exist, continue to the next {typeName}.
+  - If {type} is an object type:
+    - If {type} is a member of {possibleTypes}, add {type} to {allowedTypes}.
+  - Otherwise, if {type} is a union type:
+    - For each {concreteType} in {type}:
+      - If {concreteType} is a member of {possibleTypes}, add {concreteType} to
+        {allowedTypes}.
+  - Otherwise, if {type} is an interface type:
+    - For each {concreteType} that implements {type}:
+      - If {concreteType} is a member of {possibleTypes}, add {concreteType} to
+        {allowedTypes}.
+  - Otherwise continue to the next {typeName}.
+- Return {allowedTypes}.
+
+### Enforcing Allowed Types
+
+Enforcement of the allowed types is the responsibility of the {resolver} called
+in
+[`ResolveFieldValue()`](<https://spec.graphql.org/draft/#ResolveFieldValue()>)
+during the [`ExecuteField()`](<https://spec.graphql.org/draft/#ExecuteField()>)
+algorithm. This is because the filtering must be applied to the {collection}
+prior to applying the pagination arguments.
+
+When the field returns a list of an abstract type, the {collection} is this
+list. When the field returns a connection type over an abstract type, the
+{collection} is the list of nodes the connection represents.
+
+When a field with a `@limitTypes` argument is being resolved:
+
+- Let {limitTypesArgument} be the first argument with the `@limitTypes`
+  directive.
+- If no such argument exists, no further action is necessary.
+- If {argumentValues} does not provide a value for {limitTypesArgument}, no
+  further action is necessary.
+- Let {limitTypes} be the value in {argumentValues} of {limitTypesArgument}.
+- If {limitTypes} is {null}, no further action is necessary.
+- Let {abstractType} be the abstract type the {collection} represents.
+- Let {allowedTypes} be {CoerceAllowedTypes(abstractType, limitTypes)}.
+
+The resolver must ensure that the result of
+[`ResolveAbstractType()`](<https://spec.graphql.org/draft/#ExecuteField()>) for
+each entry in {collection} is a type within {allowedTypes}. The resolver must
+apply this restriction before applying any pagination arguments.
+
+Note: The restriction must be applied before pagination arguments so that
+non-terminal pages in the collection get full representation - i.e. there are no
+gaps.

rfcs/AbstractFilter/MatchesSpec.md

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+TODO: copy over matches spec, and update based on discussion and link to
+AbstractFilterArgumentSpec

rfcs/AbstractFilter/README.md

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+# Abstract Filter
+
+Champion: Mark Larah (@magicmark)
+
+## Overview
+
+A schema may offer a field that returns a list of an abstract type (an interface
+or union), but the user may only want a subset of these types to be returned, or
+the client may only support a subset of these types (or only current versions of
+the types, not those that are added to the union or implement the interface in
+the future). Typically, the user's preference will be that that filtering take
+place _before_ pagination arguments are applied.
+
+For example, a user is browsing a pet store, but is only interested in cats and
+fish, they want dogs to be excluded:
+
+```graphql
+{
+  allPets(first: 5, only: ["Cat", "Fish"]) {
+    name
+    price
+    ... on Cat {
+      breed
+    }
+    ... on Fish {
+      species
+    }
+  }
+}
+```
+
+Now consider widget-based user interfaces, where each concrete type has its own
+widget. This is common in server-driven UI (SDUI); for example:
+
+```graphql
+{
+  newsFeed(first: 5) {
+    ...StatusFragment
+    ...PhotoFragment
+    ...EventFragment
+  }
+}
+fragment StatusFragment on Status {
+  author {
+    name
+    avatar
+  }
+  text
+  comments(first: 2) {
+    author {
+      name
+      avatar
+    }
+    text
+  }
+}
+fragment PhotoFragment on Photo {
+  url
+  tags {
+    user {
+      name
+      avatar
+    }
+    text
+  }
+  comments(first: 2) {
+    author {
+      name
+      avatar
+    }
+    text
+  }
+}
+fragment EventFragment on Event {
+  title
+  startTime
+  duration
+  description
+  location
+  entryFee
+}
+```
+
+As the schema evolves, `newsFeed` might add a new type to the union: `Video`.
+The existing deployed client now receives empty objects where a video would be,
+and the UI no longer renders 5 tiles reliably. The client would like to filter
+to only the types it supports:
+
+```graphql
+{
+  newsFeed(first: 5, only: ["Status", "Photo", "Event"]) {
+    ...StatusFragment
+    ...PhotoFragment
+    ...EventFragment
+  }
+}
+```
+
+This RFC comes in two parts:
+
+1. An `ARGUMENT_DEFINITION` directive and associated behaviors that indicates
+   that an argument will ensure only the given types are returned
+2. A client-only `FIELD` directive that will cause this argument to be
+   automatically populated based on the types of fragments used in this field's
+   selection set
+
+## Decisions
+
+Here's a rough log of the things that we've determined so far.
+
+### String, not enum
+
+It was proposed that the `only` argument could accept an array of enum values,
+where the enum contained a value for each type the abstract type supported.
+
+Pros:
+
+- Could be auto-detected by convention (field returns list of (or connection of)
+  abstract type, argument name is `only`, argument is list of enum type, enum
+  enum contains a value for each of the possible types of the abstract type and
+  nothing else) - no need for spec changes
+- Auto-complete in GraphiQL
+- Automatically validated with errors surfaced with existing tooling already
+- Looks neat and obvious, less visual noise:
+  `{ newsFeed(only: [Status, Photo, Event]) {...} }`
+
+Cons:
+
+- Would add potentially long enums to schema (noisy)
+- Keeping enums in sync would lean towards some kind of generation in SDL-first
+  (e.g. `enum PetType @possibleTypes(typeName: "Pet")`) or dynamic construction
+  in code-first.
+- Using the type name verbatim (`GuineaPig`) would break the `UPPER_CAMEL_CASE`
+  convention, causing lint failures in some schemas
+
+The clincher that ruled out the enum type was the "Argument must accept both
+abstract and concrete types" decision - an enum composed of all of the possible
+types of an abstract type **plus** all of the abstract types that those types
+implement or are a member of would be exceedingly verbose.
+
+### Argument must accept both abstract and concrete types
+
+A "know-nothing" client (one that does not know the schema definition) would not
+know whether a fragment spread was on a concrete or abstract type, so the
+argument should support both concrete and abstract types to avoid developer
+pain.
+
+### No custom syntax
+
+Various custom syntaxes were proposed, such that the set of possible types could
+be automatically determined; for example the double-brace syntax:
+
+```graphql
+{
+  allPets {{ # e.g. resolver passed special argument `__only: ["Cat". "Dog"]`
+    Cat { ... CatFragment }
+    Dog { ... DogFragment }
+  }}
+}
+```
+
+This list of possible types would be used by the client as part of the cache key
+in the normalized store, and also would be fed to the server's resolver to
+ensure only the compatible types were returned (and GraphQL would throw an error
+if this promise were broken).
+
+All of these fell down when considering indirect relations between the selection
+set and the argument position - how would the client/schema know to feed this
+into a connection?
+
+```graphql
+{
+  allPetsConnection { # How would we know to pass the special argument here?!
+    edges {
+      cursor
+      node {{
+        Cat { ... CatFragment }
+        Dog { ... DogFragment }
+      }}
+    }
+  }
+}
+```
+
+It's essential that the client know that this filtering is applied such that its
+normalized store does not become corrupted, and thus it was agreed that we
+should stick with passing the list of allowed types as an argument (since
+arguments already factor into npormalized cache keys).
+
+### Two specifications
+
+Originally this was considered as a single feature for SDUI usage; however it
+was realised that the filter argument (and associated behavior) was useful even
+without the auto-generation of the value - the definition of behavior of such an
+argument would allow auto-completion in editors, and would be behavior the
+client could rely upon even without requiring the query be formed in a
+particular format.
+
+### Directive, not convention
+
+Clients relying on this functionality need strong guarantees. The GraphQL Cursor
+Connections Specification can rely on conventions since the requirements are
+sufficiently complex that we can be pretty sure someone is deliberately
+implementing that pattern. However, with this proposal, short of using an enum
+type (ruled out above), there's insufficient information to give us the
+confidence that the argument definitely applies the behaviors we expect.
+
+Given the above, the argument needs an annotation to indicate its special
+behavior for automated tooling to be able to rely on it, and the way to add that
+annotation currently is via a directive.
+
+It is recognized that currently this directive will not be available via
+introspection. This part of the spec therefore relies on the resolution of
+[#300](https://github.com/graphql/graphql-spec/issues/300).
+
+## Prior art
+
+Relay `@match` directive:
+https://relay.dev/docs/guides/data-driven-dependencies/server-3d/#match-design-principles
+
+PostGraphile `only` argument:
+https://github.com/graphile/crystal/blob/bcf8326bef7930b02c00b67e4ebda22a49e4f5fa/graphile-build/graphile-build-pg/src/plugins/PgPolymorphismOnlyArgumentPlugin.ts#L202-L204