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node_modules/graphql/validation/rules/SingleFieldSubscriptions.js.flow

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+// @flow strict
+
+import { GraphQLError } from '../../error/GraphQLError';
+
+import { type ASTVisitor } from '../../language/visitor';
+import { type OperationDefinitionNode } from '../../language/ast';
+
+import { type ASTValidationContext } from '../ValidationContext';
+
+export function singleFieldOnlyMessage(name: ?string): string {
+  return name
+    ? `Subscription "${name}" must select only one top level field.`
+    : 'Anonymous Subscription must select only one top level field.';
+}
+
+/**
+ * Subscriptions must only include one field.
+ *
+ * A GraphQL subscription is valid only if it contains a single root field.
+ */
+export function SingleFieldSubscriptions(
+  context: ASTValidationContext,
+): ASTVisitor {
+  return {
+    OperationDefinition(node: OperationDefinitionNode) {
+      if (node.operation === 'subscription') {
+        if (node.selectionSet.selections.length !== 1) {
+          context.reportError(
+            new GraphQLError(
+              singleFieldOnlyMessage(node.name && node.name.value),
+              node.selectionSet.selections.slice(1),
+            ),
+          );
+        }
+      }
+    },
+  };
+}