fix: Duplicate deferred fragment identifiers (#700)

Author: calvincestari

Tests/ApolloCodegenTests/CodeGeneration/Templates/DeferredFragmentsMetadataTemplateTests.swift

@@ -608,5 +608,59 @@ class DeferredFragmentsMetadataTemplateTests: XCTestCase {
       ignoringExtraLines: false)
     )
   }
-  
+
+  func test__render__givenDeferredFragmentsWithDifferentSelectionSets_doesNotRenderDuplicateIdentifiers() async throws {
+    schemaSDL = """
+      type Query {
+        allAnimals: [Animal!]
+      }
+
+      interface Animal {
+        id: String
+        species: String
+      }
+
+      type Dog implements Animal {
+        id: String
+        species: String
+      }
+      """.appendingDeferDirective()
+
+    document = """
+      query TestOperation {
+        allAnimals {
+          __typename
+          ... on Dog @defer(label: "deferredDog") {
+            id
+            ...DogFragment
+          }
+        }
+      }
+
+      fragment DogFragment on Dog {
+        species
+      }
+      """
+
+    // when
+    try await buildSubjectAndOperation()
+
+    // then
+    let rendered = renderSubject()
+
+    expect(rendered).to(equalLineByLine("""
+      enum DeferredFragmentIdentifiers {
+        static let deferredDog = DeferredFragmentIdentifier(label: "deferredDog", fieldPath: ["allAnimals"])
+      }
+      
+      public static var deferredFragments: [DeferredFragmentIdentifier: any ApolloAPI.SelectionSet.Type]? {[
+        DeferredFragmentIdentifiers.deferredDog: Data.AllAnimal.AsDog.DeferredDog.self,
+        DeferredFragmentIdentifiers.deferredDog: DogFragment.self,
+      ]}
+      """,
+      atLine: 4,
+      ignoringExtraLines: false)
+    )
+  }
+
 }

apollo-ios-codegen/Sources/ApolloCodegenLib/Templates/DeferredFragmentsMetadataTemplate.swift

@@ -44,7 +44,7 @@ struct DeferredFragmentsMetadataTemplate {
   ) -> TemplateString {
     """
     enum DeferredFragmentIdentifiers {
-    \(deferredFragmentPathTypeInfo.map {
+    \(deferredFragmentPathTypeInfo.unique(by: { $0.pathDeferConditionHash }).map {
       return """
         static let \($0.deferCondition.label) = DeferredFragmentIdentifier(label: \"\($0.deferCondition.label)\", fieldPath: [\
       \($0.path.map { "\"\($0)\"" }, separator: ", ")\
@@ -75,6 +75,22 @@ struct DeferredFragmentsMetadataTemplate {
     let path: [String]
     let deferCondition: CompilationResult.DeferCondition
     let typeName: String
+
+    /// Provides a hash value that is a combination of `path` and `deferCondition` values only, `typeName` is not
+    /// included.
+    ///
+    /// This is intended to be used when the selection set type does not matter, such as when generating a deferred
+    /// fragment identifier which is shared amongst all child selection set types within a deferred fragment.
+    ///
+    /// - Returns: Hash value of `path` and `deferCondition`.
+    var pathDeferConditionHash: Int {
+      var hasher = Hasher()
+
+      hasher.combine(path)
+      hasher.combine(deferCondition)
+
+      return hasher.finalize()
+    }
   }
 
   fileprivate func DeferredFragmentsPathTypeInfo(
@@ -134,3 +150,10 @@ struct DeferredFragmentsMetadataTemplate {
     return deferredPathTypeInfo
   }
 }
+
+fileprivate extension Sequence where Element == DeferredFragmentsMetadataTemplate.DeferredPathTypeInfo {
+  func unique<T: Hashable>(by keyForValue: (Iterator.Element) throws -> T) rethrows -> [Iterator.Element] {
+    var seen: Set<T> = []
+    return try filter { try seen.insert(keyForValue($0)).inserted }
+  }
+}