diff --git a/types/factory-girl/factory-girl-tests.ts b/types/factory-girl/factory-girl-tests.ts
index e996df56af..bda671c751 100644
--- a/types/factory-girl/factory-girl-tests.ts
+++ b/types/factory-girl/factory-girl-tests.ts
@@ -1,45 +1,66 @@
 import * as factory from "factory-girl";
 
 interface User {
-  username?: string;
-  score?: number;
-  email?: string;
-  roles?: Role[];
-  creditCard?: any;
-  boss?: User;
+    username?: string;
+    score?: number;
+    email?: string;
+    roles?: Role[];
+    creditCard?: any;
+    boss?: User;
 }
 
 interface Role {
-  id?: number;
-  name?: string;
+    id?: number;
+    name?: string;
 }
 
 interface SuperUser extends User {
-  superpower: string;
+    superpower: string;
 }
 
 // Testing setAdapter
 factory.setAdapter("my-adapter", "my-adapter-name");
 
+// Testing sequence to use it on its own
+const scoreSequence = factory.seq<number>('User.score', score => score + 1);
+
 // Testing define with seq, assoc, assocAttrs, assocMany
-factory.define<User>("user", {}, {
-  username: "Bob",
-  score: factory.seq<number>("User.score", score => score + 1),
-  email: factory.seq<string>("User.email", num => `email-${1}@users.com`),
-  roles: factory.assocMany("Role", 3, "id"),
-  creditCard: factory.assocAttrs("CreditCard", "creditCard", { number: "1234" }),
-  boss: factory.assoc("User", "boss")
-}, {
-  afterBuild: (model, attrs, options) => {},
-  afterCreate: (model, attrs, options) => {}
-});
+factory.define<User>(
+    'user',
+    {},
+    {
+        username: () => (Math.random() > 0.5 ? 'Bob' : 'Robert'),
+        score: scoreSequence,
+        email: factory.seq<string>('User.email', num => `email-${1}@users.com`),
+        roles: factory.assocMany('Role', 3, 'id'),
+        creditCard: factory.assocAttrs('CreditCard', 'creditCard', { number: '1234' }),
+        boss: factory.assoc('User', 'boss'),
+    },
+    {
+        afterBuild: (model, attrs, options) => {},
+        afterCreate: (model, attrs, options) => {},
+    }
+);
 
 // Testing extend, with and without options
 factory.extend("user", "superuser", { superpower: "flight" });
 
-factory.extend("user", "superuser", { superpower: "flight" }, {
-  afterBuild: (model, attrs, options) => {},
-  afterCreate: (model, attrs, options) => {}
+factory.extend(
+    'user',
+    'superuser',
+    { superpower: 'flight' },
+    {
+        afterBuild: (model, attrs, options) => {},
+        afterCreate: (model, attrs, options) => {},
+    }
+);
+
+factory.extend('user', 'email-related', () => {
+    const score = scoreSequence();
+    return {
+        score,
+        email: factory.seq<string>('User.email', num => `email-${num}-${score}@users.com`),
+    };
 });
 
 // Testing attrs, with and without attributes
@@ -55,40 +76,41 @@ factory.build<User>("user").then(user => user.username);
 factory.build<User>("user", { score: 10 }).then(user => user.username);
 
 // Testing buildMany, with and without attributes and options
-factory.buildMany<User>("user", 3)
-  .then(users => users.map(user => user.username));
+factory.buildMany<User>('user', 3).then(users => users.map(user => user.username));
 
-factory.buildMany<User>("user", 3, { username: "John McClane" })
-  .then(users => users.map(user => user.username));
+factory.buildMany<User>('user', 3, { username: 'John McClane' }).then(users => users.map(user => user.username));
 
 // Testing buildMany with a list of attributes
-factory.buildMany<User>("user", [
-  { username: "Jake Blues" },
-  { username: "Elwood Blues" }
-]).then(users => users.map(user => user.username));
+factory
+    .buildMany<User>('user', [{ username: 'Jake Blues' }, { username: 'Elwood Blues' }])
+    .then(users => users.map(user => user.username));
 
 // Testing create, with and without attributes
 factory.create<User>("user").then(user => user.username);
 factory.create<User>("user", { score: 10 }).then(user => user.username);
 
 // Testing createMany, with and without attributes
-factory.createMany<User>("user", 3)
-  .then(users => users.map(user => user.username));
+factory.createMany<User>('user', 3).then(users => users.map(user => user.username));
 
-factory.createMany<User>("user", 3, { username: "Rocky Balboa" })
-  .then(users => users.map(user => user.username));
+factory.createMany<User>('user', 3, { username: 'Rocky Balboa' }).then(users => users.map(user => user.username));
 
 // Testing createMany with options
-factory.createMany<User>("user", 3, { username: "John Rambo" }, {
-  afterBuild: (model, attrs, options) => {},
-  afterCreate: (model, attrs, options) => {}
-}).then(users => users.map(user => user.username));
+factory
+    .createMany<User>(
+        'user',
+        3,
+        { username: 'John Rambo' },
+        {
+            afterBuild: (model, attrs, options) => {},
+            afterCreate: (model, attrs, options) => {},
+        }
+    )
+    .then(users => users.map(user => user.username));
 
 // Testing createMany with a list of attributes
-factory.createMany<User>("user", [
-  { username: "Emmett Brown" },
-  { username: "Marty McFly" }
-]).then(users => users.map(user => user.username));
+factory
+    .createMany<User>('user', [{ username: 'Emmett Brown' }, { username: 'Marty McFly' }])
+    .then(users => users.map(user => user.username));
 
 // Testing cleanUp
 factory.cleanUp();
diff --git a/types/factory-girl/index.d.ts b/types/factory-girl/index.d.ts
index ebd6583885..01508b5ec0 100644
--- a/types/factory-girl/index.d.ts
+++ b/types/factory-girl/index.d.ts
@@ -9,86 +9,94 @@
 declare const factory: factory.Static;
 
 declare namespace factory {
-  interface Static {
-    /**
-     * Associate the factory to other model
-     */
-    assoc(model: string, attributes: string): any;
+    type Generator<T> = () => T;
 
-    /**
-     * Associate the factory to a model that's not persisted
-     */
-    assocAttrs(name: string, key?: string, attributes?: any): any;
+    type Definition<T> = T | Generator<T>;
 
-    /**
-     * Associate the factory to multiple other models
-     */
-    assocMany(model: string, num: number, attributes: string): any[];
+    type Attributes<T> = Definition<{
+        [P in keyof T]: Definition<T[P]>;
+    }>;
 
-    /**
-     * Generates and returns model attributes as an object hash instead of the model instance
-     */
-    attrs<T>(name: string, attrs?: Partial<T>): Promise<T>;
+    interface Static {
+        /**
+         * Associate the factory to other model
+         */
+        assoc(model: string, attributes: string): any;
 
-    /**
-     * Generates and returns a collection of model attributes as an object hash instead of the model instance
-     */
-    attrsMany<T>(name: string, num: number, attrs?: Array<Partial<T>>): Promise<T[]>;
+        /**
+         * Associate the factory to a model that's not persisted
+         */
+        assocAttrs(name: string, key?: string, attributes?: any): any;
 
-    /**
-     * Builds a new model instance that is not persisted
-     */
-    build<T>(name: string, attrs?: Partial<T>): Promise<T>;
+        /**
+         * Associate the factory to multiple other models
+         */
+        assocMany(model: string, num: number, attributes: string): any[];
 
-    /**
-     * Builds an array of model instances that are persisted
-     */
-    buildMany<T>(name: string, num: number, attrs?: Partial<T>): Promise<T[]>;
-    buildMany<T>(name: string, attrs?: Array<Partial<T>>): Promise<T[]>;
+        /**
+         * Generates and returns model attributes as an object hash instead of the model instance
+         */
+        attrs<T>(name: string, attrs?: Attributes<Partial<T>>): Promise<T>;
 
-    /**
-     * Destroys all of the created models
-     */
-    cleanUp(): void;
+        /**
+         * Generates and returns a collection of model attributes as an object hash instead of the model instance
+         */
+        attrsMany<T>(name: string, num: number, attrs?: ReadonlyArray<Attributes<Partial<T>>>): Promise<T[]>;
 
-    /**
-     * Builds a new model instance that is persisted
-     */
-    create<T>(name: string, attrs?: Partial<T>): Promise<T>;
+        /**
+         * Builds a new model instance that is not persisted
+         */
+        build<T>(name: string, attrs?: Attributes<Partial<T>>): Promise<T>;
 
-    /**
-     * Builds an array of model instances that are persisted
-     */
-    createMany<T>(name: string, num: number, attrs?: Partial<T>, buildOptions?: Options<T>): Promise<T[]>;
-    createMany<T>(name: string, attrs?: Array<Partial<T>>, buildOptions?: Options<T>): Promise<T[]>;
+        /**
+         * Builds an array of model instances that are persisted
+         */
+        buildMany<T>(name: string, num: number, attrs?: Attributes<Partial<T>>): Promise<T[]>;
+        buildMany<T>(name: string, attrs?: ReadonlyArray<Attributes<Partial<T>>>): Promise<T[]>;
 
-    /**
-     * Define a new factory with a set of options
-     */
-    define<T>(name: string, model: any, attrs: T, options?: Options<T>): void;
+        /**
+         * Destroys all of the created models
+         */
+        cleanUp(): void;
 
-    /**
-     * Extends a factory
-     */
-    extend(parent: string, name: string, initializer: any, options?: Options<any>): any;
+        /**
+         * Builds a new model instance that is persisted
+         */
+        create<T>(name: string, attrs?: Attributes<Partial<T>>): Promise<T>;
 
-    /**
-     * Generate values sequentially inside a factory
-     */
-    seq<T>(name: string, fn: (sequence: number) => T): T;
+        /**
+         * Builds an array of model instances that are persisted
+         */
+        createMany<T>(name: string, num: number, attrs?: Attributes<Partial<T>>, buildOptions?: Options<T>): Promise<T[]>;
+        createMany<T>(name: string, attrs?: ReadonlyArray<Attributes<Partial<T>>>, buildOptions?: Options<T>): Promise<T[]>;
 
-    /**
-     * Register an adapter, either as default or tied to a specific model
-     */
-    setAdapter(adapter: any, name?: string): void;
-  }
+        /**
+         * Define a new factory with a set of options
+         */
+        define<T>(name: string, model: any, attrs: Attributes<T>, options?: Options<T>): void;
 
-  interface Options<T> {
-    afterBuild?: Hook<T>;
-    afterCreate?: Hook<T>;
-  }
+        /**
+         * Extends a factory
+         */
+        extend(parent: string, name: string, initializer: any, options?: Options<any>): any;
 
-  type Hook<T> = (model: any, attrs: T[], options: any) => void;
+        /**
+         * Generate values sequentially inside a factory
+         */
+        seq<T>(name: string, fn: (sequence: number) => T): Generator<T>;
+
+        /**
+         * Register an adapter, either as default or tied to a specific model
+         */
+        setAdapter(adapter: any, name?: string): void;
+    }
+
+    interface Options<T> {
+        afterBuild?: Hook<T>;
+        afterCreate?: Hook<T>;
+    }
+
+    type Hook<T> = (model: any, attrs: T[], options: any) => void;
 }
 
 export = factory;