feat(AIP-122): require parent in resource name

It is a common use case to first look up the parent resource, then perform an
operation on a child. If the child does not incude the resource name of the
parent, the consumer of the resource must choose from a set of non-optimal
approaches to find the parent:

1. Perform a read of the child resource, then look up a custom field on the
   child to find the parent (assuming such a field exists).
1. Add another field where one specifies the parent of a resource in addition to
   the child resource.

Author: toumorokoshi

aip/general/0122.md

@@ -52,6 +52,8 @@ the leading slash:
     [`OUTPUT_ONLY`](./0203.md#output-only) field behavior classification.
   - Resources **must not** expose tuples, self-links, or other forms of
     resource identification.
+  - Resources **must** include their parent's resource name in their resource
+    names.
   - All ID fields **should** be strings.
 
 **Note:** Resource names as described here are used within the scope of a
@@ -322,6 +324,20 @@ message Book {
 }
 ```
 
+## Rationale
+
+### Including the parent resource name in the resource name
+
+It is a common use case to first look up the parent resource, then perform an
+operation on a child. If the child does not incude the resource name of the
+parent, the consumer of the resource must choose from a set of non-optimal
+approaches to find the parent:
+
+1. Perform a read of the child resource, then look up a custom field on the
+   child to find the parent (assuming such a field exists).
+1. Add another field where one specifies the parent of a resource in addition to
+   the child resource.
+
 ## Further reading
 
 - For evolving resource names over time, see