graphql · benjie · Feb 19, 2021 · Feb 19, 2021 · Feb 19, 2021 · Feb 26, 2021
diff --git a/cspell.yml b/cspell.yml
@@ -21,6 +21,8 @@ words:
   - tatooine
   - zuck
   - zuckerberg
+  - brontie
+  - oneOf
 # Forbid Alternative spellings
 flagWords:
   - implementor

diff --git a/spec/Section 3 -- Type System.md b/spec/Section 3 -- Type System.md
@@ -1568,6 +1568,28 @@ define arguments or contain references to interfaces and unions, neither of
 which is appropriate for use as an input argument. For this reason, input
 objects have a separate type in the system.
 
+**OneOf Input Objects**
+
+OneOf Input Objects are a special variant of Input Objects where the type system
+asserts that exactly one of the fields must be set and non-null, all others
+being omitted. This is useful for representing situations where an input may be
+one of many different options.
+
+When using the type system definition language, the `@oneOf` directive is used
+to indicate that an Input Object is a OneOf Input Object (and thus requires
+exactly one of its field be provided):
+
+```graphql
+input UserUniqueCondition @oneOf {
+  id: ID
+  username: String
+  organizationAndEmail: OrganizationAndEmailInput
+}
+```
+
+In schema introspection, the `__Type.isOneOf` field will return {true} for OneOf
+Input Objects, and {false} for all other Input Objects.
+
 **Circular References**
 
 Input Objects are allowed to reference other Input Objects as field types. A
@@ -1661,6 +1683,21 @@ is constructed with the following rules:
   variable definition does not provide a default value, the input object field
   definition's default value should be used.
 
+Further, if the input object is a OneOf Input Object, the following additional
+rules apply:
+
+- If the input object literal or unordered map does not contain exactly one
+  entry an _execution error_ must be raised.
+
+- Within the input object literal or unordered map, if the single entry is the
+  {null} literal or {null} an _execution error_ must be raised.
+
+- If the coerced unordered map does not contain exactly one entry an _execution
+  error_ must be raised.
+
+- If the value of the single entry in the coerced unordered map is {null} an
+  _execution error_ must be raised.
+
 Following are examples of input coercion for an input object type with a
 `String` field `a` and a required (non-null) `Int!` field `b`:
 
@@ -1690,6 +1727,46 @@ input ExampleInputObject {
 | `{ b: $var }`            | `{ var: null }`         | Error: {b} must be non-null.         |
 | `{ b: 123, c: "xyz" }`   | `{}`                    | Error: Unexpected field {c}          |
 
+Following are examples of input coercion for a OneOf Input Object type with a
+`String` member field `a` and an `Int` member field `b`:
+
+```graphql example
+input ExampleInputTagged @oneOf {
+  a: String
+  b: Int
+}
+```
+
+| Literal Value            | Variables                        | Coerced Value                                       |
+| ------------------------ | -------------------------------- | --------------------------------------------------- |
+| `{ a: "abc", b: 123 }`   | `{}`                             | Error: Exactly one key must be specified            |
+| `{ a: null, b: 123 }`    | `{}`                             | Error: Exactly one key must be specified            |
+| `{ a: null, b: null }`   | `{}`                             | Error: Exactly one key must be specified            |
+| `{ a: null }`            | `{}`                             | Error: Value for member field {a} must be non-null  |
+| `{ b: 123 }`             | `{}`                             | `{ b: 123 }`                                        |
+| `{}`                     | `{}`                             | Error: Exactly one key must be specified            |
+| `{ a: $a, b: 123 }`      | `{ a: null }`                    | Error: Exactly one key must be specified            |
+| `{ a: $a, b: 123 }`      | `{}`                             | Error: Exactly one key must be specified            |
+| `{ a: $a, b: $b }`       | `{ a: "abc" }`                   | Error: Exactly one key must be specified            |
+| `{ b: $b }`              | `{ b: 123 }`                     | `{ b: 123 }`                                        |
+| `$var`                   | `{ var: { b: 123 } }`            | `{ b: 123 }`                                        |
+| `$var`                   | `{ var: { a: "abc", b: 123 } }`  | Error: Exactly one key must be specified            |
+| `$var`                   | `{ var: { a: "abc", b: null } }` | Error: Exactly one key must be specified            |
+| `$var`                   | `{ var: { a: null } }`           | Error: Value for member field {a} must be non-null  |
+| `$var`                   | `{ var: {} }`                    | Error: Exactly one key must be specified            |
+| `"abc123"`               | `{}`                             | Error: Incorrect value                              |
+| `$var`                   | `{ var: "abc123" } }`            | Error: Incorrect value                              |
+| `{ a: "abc", b: "123" }` | `{}`                             | Error: Exactly one key must be specified            |
+| `{ b: "123" }`           | `{}`                             | Error: Incorrect value for member field {b}         |
+| `$var`                   | `{ var: { b: "abc" } }`          | Error: Incorrect value for member field {b}         |
+| `{ a: "abc" }`           | `{}`                             | `{ a: "abc" }`                                      |
+| `{ b: $b }`              | `{}`                             | Error: Value for member field {b} must be specified |
+| `$var`                   | `{ var: { a: "abc" } }`          | `{ a: "abc" }`                                      |
+| `{ a: "abc", b: null }`  | `{}`                             | Error: Exactly one key must be specified            |
+| `{ b: $b }`              | `{ b: null }`                    | Error: Value for member field {b} must be non-null  |
+| `{ b: 123, c: "xyz" }`   | `{}`                             | Error: Unexpected field {c}                         |
+| `$var`                   | `{ var: { b: 123, c: "xyz" } }`  | Error: Unexpected field {c}                         |
+
 **Type Validation**
 
 1. An Input Object type must define one or more input fields.
@@ -1702,6 +1779,9 @@ input ExampleInputObject {
       returns {true}.
    4. If input field type is Non-Null and a default value is not defined:
       1. The `@deprecated` directive must not be applied to this input field.
+   5. If the Input Object is a OneOf Input Object then:
+      1. The type of the input field must be nullable.
+      2. The input field must not have a default value.
 3. If an Input Object references itself either directly or through referenced
    Input Objects, at least one of the fields in the chain of references must be
    either a nullable or a List type.
@@ -1729,6 +1809,12 @@ defined.
    the previous Input Object.
 4. Any non-repeatable directives provided must not already apply to the previous
    Input Object type.
+5. The `@oneOf` directive must not be provided by an Input Object type
+   extension.
+6. If the original Input Object is a OneOf Input Object then:
+   1. All fields of the Input Object type extension must be nullable.
+   2. All fields of the Input Object type extension must not have default
+      values.
 
 ## List
 
@@ -1954,6 +2040,9 @@ schema.
 GraphQL implementations that support the type system definition language should
 provide the `@specifiedBy` directive if representing custom scalar definitions.
 
+GraphQL implementations that support the type system definition language should
+provide the `@oneOf` directive if representing OneOf Input Objects.
+
 When representing a GraphQL schema using the type system definition language any
 _built-in directive_ may be omitted for brevity.
 
@@ -2168,3 +2257,20 @@ to the relevant IETF specification.
 ```graphql example
 scalar UUID @specifiedBy(url: "https://tools.ietf.org/html/rfc4122")
 ```
+
+### @oneOf
+
+```graphql
+directive @oneOf on INPUT_OBJECT
+```
+
+The `@oneOf` _built-in directive_ is used within the type system definition
+language to indicate an Input Object is a OneOf Input Object.
+
+```graphql example
+input UserUniqueCondition @oneOf {
+  id: ID
+  username: String
+  organizationAndEmail: OrganizationAndEmailInput
+}
+```
diff --git a/spec/Section 4 -- Introspection.md b/spec/Section 4 -- Introspection.md
@@ -151,6 +151,8 @@ type __Type {
   ofType: __Type
   # may be non-null for custom SCALAR, otherwise null.
   specifiedByURL: String
+  # should be non-null for INPUT_OBJECT only
+  isOneOf: Boolean
 }
 
 enum __TypeKind {
@@ -373,6 +375,8 @@ Fields\:
 - `inputFields` must return the set of input fields as a list of `__InputValue`.
   - Accepts the argument `includeDeprecated` which defaults to {false}. If
     {true}, deprecated input fields are also returned.
+- `isOneOf` must return {true} when representing a OneOf Input Object, {false}
+  otherwise.
 - All other fields must return {null}.
 
 **List**

diff --git a/spec/Section 5 -- Validation.md b/spec/Section 5 -- Validation.md
@@ -41,6 +41,11 @@ type Query {
   findDog(searchBy: FindDogInput): Dog
 }
 
+type Mutation {
+  addPet(pet: PetInput!): Pet
+  addPets(pet: [PetInput!]!): [Pet]
+}
+
 enum DogCommand {
   SIT
   DOWN
@@ -93,6 +98,23 @@ input FindDogInput {
   name: String
   owner: String
 }
+
+input CatInput {
+  name: String!
+  nickname: String
+  meowVolume: Int
+}
+
+input DogInput {
+  name: String!
+  nickname: String
+  barkVolume: Int
+}
+
+input PetInput @oneOf {
+  cat: CatInput
+  dog: DogInput
+}
 ```
 
 ## Documents
@@ -1346,6 +1368,12 @@ query goodComplexDefaultValue($search: FindDogInput = { name: "Fido" }) {
     name
   }
 }
+
+mutation addPet($pet: PetInput! = { cat: { name: "Brontie" } }) {
+  addPet(pet: $pet) {
+    name
+  }
+}
 ```
 
 Non-coercible values (such as a String into an Int) are invalid. The following
@@ -1361,6 +1389,24 @@ query badComplexValue {
     name
   }
 }
+
+mutation oneOfWithNoFields {
+  addPet(pet: {}) {
+    name
+  }
+}
+
+mutation oneOfWithTwoFields($dog: DogInput) {
+  addPet(pet: { cat: { name: "Brontie" }, dog: $dog }) {
+    name
+  }
+}
+
+mutation listOfOneOfWithNullableVariable($dog: DogInput) {
+  addPets(pets: [{ dog: $dog }]) {
+    name
+  }
+}
 ```
 
 ### Input Object Field Names
@@ -1887,8 +1933,8 @@ IsVariableUsageAllowed(variableDefinition, variableUsage):
 - Let {variableType} be the expected type of {variableDefinition}.
 - Let {locationType} be the expected type of the {Argument}, {ObjectField}, or
   {ListValue} entry where {variableUsage} is located.
-- If {locationType} is a non-null type AND {variableType} is NOT a non-null
-  type:
+- If {IsNonNullPosition(locationType, variableUsage)} AND {variableType} is NOT
+  a non-null type:
   - Let {hasNonNullVariableDefaultValue} be {true} if a default value exists for
     {variableDefinition} and is not the value {null}.
   - Let {hasLocationDefaultValue} be {true} if a default value exists for the
@@ -1899,6 +1945,15 @@ IsVariableUsageAllowed(variableDefinition, variableUsage):
   - Return {AreTypesCompatible(variableType, nullableLocationType)}.
 - Return {AreTypesCompatible(variableType, locationType)}.
 
+IsNonNullPosition(locationType, variableUsage):
+
+- If {locationType} is a non-null type, return {true}.
+- If the location of {variableUsage} is an {ObjectField}:
+  - Let {parentLocationType} be the expected type of {ObjectField}'s parent
+    {ObjectValue}.
+  - If {parentLocationType} is a OneOf Input Object type, return {true}.
+- Return {false}.
+
 AreTypesCompatible(variableType, locationType):
 
 - If {locationType} is a non-null type:
@@ -1987,6 +2042,34 @@ query listToNonNullList($booleanList: [Boolean]) {
 This would fail validation because a `[T]` cannot be passed to a `[T]!`.
 Similarly a `[T]` cannot be passed to a `[T!]`.
 
+Variables used for OneOf Input Object fields must be non-nullable.
+
+```graphql example
+mutation addCat($cat: CatInput!) {
+  addPet(pet: { cat: $cat }) {
+    name
+  }
+}
+mutation addCatWithDefault($cat: CatInput! = { name: "Brontie" }) {
+  addPet(pet: { cat: $cat }) {
+    name
+  }
+}
+```
+
+```graphql counter-example
+mutation addNullableCat($cat: CatInput) {
+  addPet(pet: { cat: $cat }) {
+    name
+  }
+}
+mutation addNullableCatWithDefault($cat: CatInput = { name: "Brontie" }) {
+  addPet(pet: { cat: $cat }) {
+    name
+  }
+}
+```
+
 **Allowing Optional Variables When Default Values Exist**
 
 A notable exception to typical variable type compatibility is allowing a