docs: include filter

docs/site/Include-filter.md

@@ -7,4 +7,229 @@ source: loopback-next
 file: packages/metadata/README.md
 sidebar: lb4_sidebar
 permalink: /doc/en/lb4/Include-filter.html
----
+---
+
+An _include_ filter enables you to include results from related models in a query over relations. (See [Relations](Relations.md) for details of defining relations)
+
+The value of the include filter can be a string, an array, or an object.
+
+{% include important.html content="You can use an _include_ filter with `find(),` `findOne()` and `findById()` methods.
+" %}
+
+### Node API
+
+{% include content/angular-methods-caveat.html lang=page.lang %}
+
+To query one relation:
+```ts
+{include: [{relation: 'relationName'}]}
+```
+
+To query multiple relations:
+```ts
+{include: [{relation: 'relationName1'}, {relation: 'relationName2'}]}
+```
+
+To query nested relations, use the scope field:
+
+```ts
+{
+    relation: 'relationName',
+    scope: {
+    include: [{relation: 'nestedRelationName'}],
+    },
+}
+```
+
+Where:
+
+- _relationName_ is the name of a relation defined in repositories. Check [Relations](Relations.md) for details.
+
+### REST API
+
+To query one relation:
+`/modelName?filter[include][][relation]=_relationName_`
+
+To query multiple relations:
+`/modelName?filter[include][0][relation]=_relationName1_&filter[include][1][relation]=_relationName2_`
+
+To query nested relations, as the url would get too long, we recommend to encode it with `encodeURIComponent(JSON.stringify(filter))`:
+`/modelName?filter=<encodeResult>`
+
+### Examples
+
+- Return all customers with their orders:
+
+```ts
+await customerRepository.find({include: [{relation: 'orders'}]});
+```
+
+This is equivalent to
+
+```
+GET /customers?filter[include][][relation]=orders
+```
+
+Result:
+
+```ts
+[{
+    id: 1,
+    name: 'Tom Nook',
+    orders:[{id: 1, desc: 'pancil'}]
+ },
+ {...}
+]
+```
+
+- Return all customers with their orders and their address:
+
+```ts
+await customerRepository.find({include: [{relation: 'orders'}, {relation: 'address'}]});
+```
+
+This is equivalent to
+
+```
+GET /customers?filter[include][0][relation]=orders?filter[include][1][relation]=address
+```
+
+
+Result:
+
+```ts
+[{
+    id: 1,
+    name: 'Tom Nook',
+    orders:[{id: 1, desc: 'pancil'}],
+    address: {'8200 Warden Ave'}
+ },
+ {...}
+]
+```
+
+- Return all customers with their orders and also the shipment information of orders:
+
+```ts
+await customerRepository.find({
+  include: [
+    {
+      relation: 'orders',
+      scope: {
+        include: [{relation: 'shipment'}],
+      },
+    },
+  ],
+});
+```
+
+This is equivalent to (using `encodeURIComponent`):
+
+```
+GET /customers?filter=%7B"include"%3A%5B%7B"relation"%3A"orders"%2C"scope"%3A%7B"include"%3A%5B%7B"relation"%3A"shipment"%7D%5D%7D%7D%5D%7D
+```
+
+Result:
+
+```ts
+[{
+    id: 1,
+    name: 'Tom Nook',
+    orders:[
+        {id: 123,
+        desc: 'pancil',
+        shipment: {id: 999, company: 'UPS'} // nested related models
+        }
+    ],
+ },
+ {...}
+]
+```
+
+#### Combined use of `fields` and `include` for a `belongsTo` relation
+
+If you want to use both `include` and `fields` to display only specific fields of a model and a specific belongsTo relation, **you need to add the relation foreign key in the `fields`** :
+
+Return all posts only with field title and the relation category:
+```ts
+await postRepository.find({include: [{relation: 'category'}], fields: ['title', 'categoryId'});
+```
+
+#### Include with filters
+
+In some cases, you may want to apply filters to related models to be included.
+
+{% include note.html content="
+
+When you apply filters to related models, the query returns results from the first model plus any results from related models with the filter query,
+similar to a \"left join\" in SQL.
+
+" %}
+
+LoopBack supports that with the following syntax (for example):
+
+```ts
+await postRepository.find({
+  include: [{
+    relation: 'owner', // include the owner object
+    scope: { // further filter the owner object
+      fields: ['username', 'email'], // only show two fields
+      include: { // include orders for the owner
+        relation: 'orders', 
+        scope: {
+          where: {orderId: 5} // only select order with id 5
+        }
+      }
+    }
+  }]
+  });
+```
+
+For real-world scenarios where only users in `$authenticated` or `$owner` roles should have access, use `findById()`.
+For example, the following example uses filters to perform pagination:
+
+```ts
+await postRepository.findById('123', {
+  include: [{
+    relation: 'owner',
+    scope: { // fetch 1st "page" with 5 entries in it
+      skip:0,
+      limit:5
+    }
+  }]
+});
+```
+
+#### Access included objects
+
+In the Node.js API, call `toJSON()` to convert the returned model instance with related items into a plain JSON object. For example:
+
+```ts
+const result = await postRepository.find({include: [{relation: 'owner'}, {relation: 'orders'}]});
+console.log(result.owner.posts, result.owner.orders);
+ //... 
+```
+
+Note the relation properties such as `post.owner` reference a JavaScript **function** for the relation method.
+
+#### REST examples
+
+These examples assume a customer model with a hasMany relationship to a reviews model. 
+
+Return all customers including their reviews:
+
+`/customers?filter[include][][relation]=reviews`
+
+Return all customers including their reviews and also their orders:
+
+`GET /`customers?filter[include][0][relation]=reviews?filter[include][1][relation]=orders`
+
+Return all customers whose age is 21, including their reviews:
+
+`/customers?filter[include][][relation]=reviews&filter[where][age]=21`
+
+Return first two customers including their reviews:
+
+`/customers?filter[include][][relation]=reviews&filter[limit]=2`
+
+**See also**: [Querying related models](HasMany-relation.md#querying-related-models).