Add support for @override directive

Author: Sylvain Cleymans

README.md

@@ -25,6 +25,14 @@ We use `@fake` directive to let you specify how to fake data. And if 60+ fakers
       pets: [Pet] @listLength(min: 1, max: 10)
     }
 
+The `@override` directive can also be used to specify that an existing field should be overriden:
+
+```graphql
+extend type Post {
+  acticle: String! @override @fake(type: lorem, options: {loremSize: paragraphs})
+}
+```
+
 No need to remember or read any docs. Autocompletion is included!
 
 ## Features

src/fake_definition.ts

@@ -12,6 +12,7 @@ import {
   isObjectType,
   isInterfaceType,
   ValuesOfCorrectTypeRule,
+  DefinitionNode,
 } from 'graphql';
 
 // FIXME
@@ -220,6 +221,8 @@ const fakeDefinitionAST = parse(/* GraphQL */ `
 
   scalar examples__JSON
   directive @examples(values: [examples__JSON]!) on FIELD_DEFINITION | SCALAR
+
+  directive @override on FIELD_DEFINITION
 `);
 
 function defToName(defNode) {
@@ -256,6 +259,14 @@ export function buildWithFakeDefinitions(
     }),
   };
 
+  if (extensionSDL != null) {
+    // Remove fields marked with the @override annotation
+    filteredAST.definitions = filterFields(
+      filteredAST.definitions,
+      findFieldsWithOverrideDirective(extensionSDL),
+    );
+  }
+
   let schema = extendSchemaWithAST(schemaWithOnlyFakedDefinitions, filteredAST);
 
   const config = schema.toConfig();
@@ -311,6 +322,54 @@ export function buildWithFakeDefinitions(
       commentDescriptions: true,
     });
   }
+
+  function findFieldsWithOverrideDirective(
+    extensionSDL: Source,
+  ): Map<string, string[]> {
+    const res = new Map();
+    parseSDL(extensionSDL).definitions.map((d) => {
+      if (d.kind !== 'ObjectTypeExtension') {
+        return;
+      }
+
+      const filteredFields = d
+        .fields!.filter(
+          (f) =>
+            f.directives!.findIndex((d) => d.name.value === 'override') !== -1,
+        )
+        .map((f) => f.name.value);
+
+      if (filteredFields.length > 0) {
+        res.set(d.name.value, filteredFields);
+      }
+    });
+
+    return res;
+  }
+
+  function filterFields(
+    definitions: DefinitionNode[],
+    fieldsToRemove: Map<string, string[]>,
+  ): DefinitionNode[] {
+    return definitions.map((d) => {
+      if (
+        d.kind === 'ObjectTypeDefinition' &&
+        fieldsToRemove.has(d.name.value)
+      ) {
+        const toRemove = fieldsToRemove.get(d.name.value);
+        const filteredFields = d.fields!.filter(
+          (f) => toRemove!.indexOf(f.name.value) === -1,
+        );
+
+        return {
+          ...d,
+          fields: filteredFields,
+        };
+      }
+
+      return d;
+    });
+  }
 }
 
 // FIXME: move to 'graphql-js'