New test data models documentation (#687)

* docs: test data models overview document

* docs: include new test data model guidelines docs

* fix(docs): apply first feedback for data models overview doc

* fix(docs): apply first feedback for building data model doc

* docs: apply feedback

* docs: apply feedback

* docs: fix wording

* docs: apply pr feedback

CONTRIBUTING.md

@@ -33,6 +33,10 @@ To know more about how to work and build data models, [check out the documentati
 
 Once it's done, you can run `pnpm test` to test your changes.
 
+## Building a new test data model
+
+Please refer to [this documentation](docs/guidelines/creating-new-model.md) to get detailed information.
+
 ## Adding changesets
 
 commercetools test-data uses [changesets](https://github.com/atlassian/changesets) to do versioning and creating changelogs.

docs/contributing/test-data-models-overview.md

@@ -0,0 +1,250 @@
+# Test data models overview
+
+A test data model is set of helper objects commercetools API consumers can use when writing their own tests in order to mock the data returned by commercetools services.
+
+These helpers are flexible enough to represent different types of responses from different APIs and with different values.
+
+## Public API
+
+Each test data model has the same API which consists of:
+
+- A Rest builder
+- A GraphQL builder
+- TypeScript types
+
+A `Builder` is the object you are interacting with as a consumer. It has only two functions:
+
+- `random`: exposes a fluent API which allows to build a customized version of the data model
+- `presets`: predefined versions of the data model
+
+This is how an entry point of a data model package looks like:
+
+```ts
+import { RestModelBuilder, GraphqlModelBuilder } from './builders';
+import * as GeometryPresets from './presets';
+
+export * from './types';
+
+export const GeometryRest = {
+  random: RestModelBuilder,
+  presets: GeometryPresets.restPresets,
+};
+
+export const GeometryGraphql = {
+  random: GraphqlModelBuilder,
+  presets: GeometryPresets.graphqlPresets,
+};
+```
+
+### Examples
+
+#### Building default random data models
+
+```ts
+import {
+  RestModelBuilder,
+  GraphqlModelBuilder,
+  type TGeometryRest,
+  type TGeometryGraphql,
+} from '@commercetools-test-data/geometry';
+
+const restGeometry: TGeometryRest = RestModelBuilder.random().build();
+const graphqlGeometry: TGeometryGraphql = GraphqlModelBuilder.random().build();
+```
+
+#### Building customized data models
+
+```ts
+import {
+  RestModelBuilder,
+  GraphqlModelBuilder,
+  type TGeometryRest,
+  type TGeometryGraphql,
+} from '@commercetools-test-data/geometry';
+
+const restGeometry: TGeometryRest = RestModelBuilder.random()
+  .coordinates([12.45, 66.33])
+  .build();
+const graphqlGeometry: TGeometryGraphql = GraphqlModelBuilder.random()
+  .coordinates([12.45, 66.33])
+  .build();
+```
+
+#### Building predefined version data models
+
+```ts
+import {
+  RestModelBuilder,
+  GraphqlModelBuilder,
+  type TGeometryRest,
+  type TGeometryGraphql,
+} from '@commercetools-test-data/geometry';
+
+const restGeometry: TGeometryRest = RestModelBuilder.presets.munich().build();
+const graphqlGeometry: TGeometryGraphql = GraphqlModelBuilder.presets
+  .munich()
+  .build();
+```
+
+## Internal structure
+
+Test data model are implemented using some classes and helpers exposed from the [core](https://github.com/commercetools/test-data/tree/main/core) package.
+
+### Fields config
+
+The core file is named `fields-config` and its responsibility is to describe how the data model properties should be pre-populated and also resolve dependencies among them (if any).
+
+Here's an example of a simple one:
+
+```ts
+import { fake, type TModelFieldsConfig } from '@commercetools-test-data/core';
+import type { TGeometryGraphql, TGeometryRest } from './types';
+
+const commonFieldsConfig = {
+  type: fake(() => 'Point'),
+  coordinates: fake((f) => [f.location.longitude(), f.location.latitude()]),
+};
+
+export const restFieldsConfig: TModelFieldsConfig<TGeometryRest> = {
+  fields: {
+    ...commonFieldsConfig,
+  },
+};
+
+export const graphqlFieldsConfig: TModelFieldsConfig<TGeometryGraphql> = {
+  fields: {
+    ...commonFieldsConfig,
+    __typename: 'Geometry',
+  },
+};
+```
+
+As you can see, these files are required to export two configuration objects (`restFieldsConfig`, `graphqlFieldsConfig`) which describe how to populate the properties of the data model when building the default version.
+
+The configuration objects have two properties:
+
+- `fields` (**required**): Describe how the default values should be populated
+- `postBuild` (_optional_): Callback function to run modifications on the built data model
+
+Here's an example where we use `postBuild` option to populate some values which depend on others to be built first:
+
+```ts
+export const graphqlFieldsConfig: TModelFieldsConfig<TChannelGraphql> = {
+  fields: {
+    ...commonFieldsConfig,
+    __typename: 'Channel',
+    nameAllLocales: null,
+    descriptionAllLocales: null,
+  },
+  postBuild: (model) => {
+    const name = model.nameAllLocales
+      ? LocalizedString.resolveGraphqlDefaultLocaleValue(model.nameAllLocales)
+      : undefined;
+    const description = model.descriptionAllLocales
+      ? LocalizedString.resolveGraphqlDefaultLocaleValue(
+          model.descriptionAllLocales
+        )
+      : undefined;
+    return {
+      name,
+      description,
+    };
+  },
+};
+```
+
+In this case, `name` and `description` depend on the values from `nameAllLocales` and `descriptionAllLocales` but we don't know them in advance, only once the data model has been built. The `postBuild` callback allows to manipulate the built object before it's returned to the consumer.
+
+This function just need to return an object with the updated properties and their values and it will be merged with the originally built one.
+
+### Builders
+
+The `builders` is another main file in which we actually create the objects which implement the fluent API that allows generating data models objects.
+
+This is how it looks like:
+
+```ts
+import { createSpecializedBuilder } from '@commercetools-test-data/core';
+import { restFieldsConfig, graphqlFieldsConfig } from './fields-config';
+import type {
+  TCreateGeometryBuilder,
+  TGeometryGraphql,
+  TGeometryRest,
+} from './types';
+
+export const RestModelBuilder: TCreateGeometryBuilder<TGeometryRest> = () =>
+  createSpecializedBuilder({
+    name: 'GeometryRestBuilder',
+    type: 'rest',
+    modelFieldsConfig: restFieldsConfig,
+  });
+
+export const GraphqlModelBuilder: TCreateGeometryBuilder<
+  TGeometryGraphql
+> = () =>
+  createSpecializedBuilder({
+    name: 'GeometryGraphqlBuilder',
+    type: 'graphql',
+    modelFieldsConfig: graphqlFieldsConfig,
+  });
+```
+
+We're exporting one builder for each representation (REST and GraphQL) of the data models for each of which we use the respective `fields-config` file detailed above.
+
+## Testing
+
+It's very important we implement relevant tests for our data models builders so we can make sure consumers will be creating the right data out of our exported assets.
+
+The bare minimum tests to implement are those which validate the generated default models contain the expected base data.
+
+Here's an example:
+
+```ts
+import { GeometryRest, GeometryGraphql } from './index';
+
+describe('Geometry Builder', () => {
+  it('should build properties for the REST representation', () => {
+    const restModel = GeometryRest.random().build();
+
+    expect(restModel).toEqual(
+      expect.objectContaining({
+        type: 'Point',
+        coordinates: expect.arrayContaining([
+          expect.any(Number),
+          expect.any(Number),
+        ]),
+      })
+    );
+  });
+  it('should build properties for the GraphQL representation', () => {
+    const graphqlModel = GeometryGraphql.random().build();
+
+    expect(graphqlModel).toEqual(
+      expect.objectContaining({
+        type: 'Point',
+        coordinates: expect.arrayContaining([
+          expect.any(Number),
+          expect.any(Number),
+        ]),
+        __typename: 'Geometry',
+      })
+    );
+  });
+});
+```
+
+It's also recommended to test a customized version of the data models as well:
+
+```ts
+it('should build a customized version of the GraphQL data model', () => {
+  const graphqlModel = GeometryGraphql.random().coordinates([10, 20]).build();
+
+  expect(graphqlModel).toEqual(
+    expect.objectContaining({
+      type: 'Point',
+      coordinates: expect.arrayContaining([10, 20]),
+      __typename: 'Geometry',
+    })
+  );
+});
+```