Merge pull request rails#33385 from lanzhiheng/add-example-for-has-and-belongs-to-many-association

 [ci skip]  Add example for `has-and-belongs-to-many` association.

Author: rafaelfranca

guides/source/association_basics.md

@@ -365,6 +365,22 @@ class CreateAssembliesAndParts < ActiveRecord::Migration[5.0]
 end
 ```
 
+As you can see we don't create the relevant model for `assemblies_parts` table, So We can't create the many-to-many record like this
+
+``` ruby
+AssemblyPart.create(assembly: @assembly, part: @part) # => NameError: uninitialized constant AssemblyPart
+```
+
+If you want to create the relevant many-to-many record, you can use the below code
+
+``` ruby
+@assembly.parts << @part
+# Or
+@part.assemblies << @assembly
+```
+
+They are equivalent.
+
 ### Choosing Between `belongs_to` and `has_one`
 
 If you want to set up a one-to-one relationship between two models, you'll need to add `belongs_to` to one, and `has_one` to the other. How do you know which is which?