Add guide on how to scope has_many relations

docs/guides.md

@@ -2,5 +2,6 @@
 title: Guides
 ---
 
-* [Hiding Dashboards from the Sidebar](./guides/hiding_dashboards_from_sidebar)
-* [Customising the search](./guides/customising_search)
+- [Hiding Dashboards from the Sidebar](./guides/hiding_dashboards_from_sidebar)
+- [Customising the search](./guides/customising_search)
+- [Scoping HasMany Relations](./guides/scoping_has_many_relations.md)

docs/guides/scoping_has_many_relations.md

@@ -0,0 +1,27 @@
+---
+title: Scoping HasMany Relations
+---
+
+To show a subset of a has_many relationship, create a new [has_many](https://apidock.com/rails/ActiveRecord/Associations/ClassMethods/has_many) relationship in your model (using the `scope` argument) and add it to the model's dashboard.
+
+## Creating a scoped has_many relationship
+
+Models can define subsets of a `has_many` relationship by passing a callable (i.e. proc or lambda) as its second argument.
+
+```ruby
+class Customer < ApplicationRecord
+   has_many :orders
+   has_many :processed_orders, ->{ where(processed: true) }, class_name: "Order"
+```
+
+Since ActiveRecord infers the class name from the first argument, the new `has_many` relation needs to specify the model using the `class_name` option.
+
+## Add new relationship to dashboard
+
+Your new scoped relation can be used in the dashboard just like the original `HasMany`. Notice the new field needs to specifiy the class name as an option like you did in the model.
+
+```ruby
+ATTRIBUTE_TYPES = {
+  orders: Field::HasMany,
+  processed_orders: Field::HasMany.with_options(class_name: 'Order')
+```