diff --git a/aip/general/0124/aip.md.j2 b/aip/general/0124/aip.md.j2
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+# Resource association
+
+APIs sometimes have resource hierarchies that can not be cleanly expressed in
+the usual tree structure. For example, a resource may have a many-to-one
+relationship with two other resource types instead of just one. Alternatively,
+a resource may have a many-to-many relationship with another resource type.
+
+## Guidance
+
+A resource **must** have at most one canonical parent, and `GET` operations to
+read collections **must not** require two distinct "parents" to work.
+
+### Multiple many-to-one associations
+
+If a resource has a many-to-one relationship with multiple resource types, it
+**must** choose at most one of them to be the canonical parent. The resource
+**may** be associated with other resources through other fields on the
+resource.
+
+{% tab proto -%}
+
+{% sample 'many_to_one.proto', 'message Book' %}
+
+{% endtabs %}
+
+When listing resources (see AIP-132) with multiple associations in this way,
+the RPC **must** treat the `string parent` field, and **must not** add
+additional required arguments; additionally, the operation **should** include a
+`string filter` field (see AIP-160) that allows users to filter by other
+resource associations.
+
+### Many-to-many associations
+
+Many-to-many associations are less common in APIs than they are in relational
+databases, in part because they are more difficult to model and present over
+network interfaces.
+
+An API **may** contain many-to-many relationships, and **should** use a
+repeated field containing a list of resource names, following the principles
+described for repeated fields in AIP-144.
+
+{% tab proto -%}
+
+{% sample 'many_to_many.proto', 'message Book' %}
+
+{% endtabs %}
+
+**Note:** See AIP-144 for more information on repeated fields, including how to
+handle common issues such as atomic changes.
+
+If the use of a repeated field is too restrictive, or if more metadata is
+required along with the association, an API **may** model a many-to-many
+relationship using a sub-resource with two one-to-many associations.
+
+{% tab proto -%}
+
+{% sample 'subresource.proto', 'message BookAuthor' %}
+
+{% endtabs %}
+
+**Note:** Using subresources to model an association between resources is only
+recommended if additional metadata is required in the relationship, or if the
+restrictions around the use of a repeated field preclude the use of that
+approach.
diff --git a/aip/general/0124/aip.yaml b/aip/general/0124/aip.yaml
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+++ b/aip/general/0124/aip.yaml
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+---
+id: 124
+state: approved
+created: 2020-03-20
+placement:
+  category: resource-design
+  order: 40
diff --git a/aip/general/0124/many_to_many.proto b/aip/general/0124/many_to_many.proto
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+++ b/aip/general/0124/many_to_many.proto
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+// Copyright 2021 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto3";
+
+import "google/api/resource.proto";
+
+// A representation of an individual person who writes a book.
+message Author {
+  option (google.api.resource) = {
+    type: "library.googleapis.com/Author"
+    pattern: "publishers/{publisher}/authors/{author}"
+  };
+
+  // The name of the author.
+  // Format: publishers/{publisher}/authors/{author}
+  string name = 1;
+
+  // The author's given name (in Western culture: "first name").
+  string given_name = 2;
+
+  // The author's family name (in Western culture: "last name").
+  string family_name = 3;
+}
+
+// A representation of a single book.
+message Book {
+  option (google.api.resource) = {
+    type: "library.googleapis.com/Book"
+    pattern: "publishers/{publisher}/books/{book}"
+  };
+
+  // The name of the book.
+  // Format: publishers/{publisher}/books/{book}
+  string name = 1;
+
+  // The resource names for the authors of the book.
+  // Format: publishers/{publisher}/authors/{author}
+  repeated string authors = 2 [(google.api.resource_reference) = {
+    type: "library.googleapis.com/Author"
+  }];
+
+  // The rating assigned to the book.
+  float rating = 3;
+}
diff --git a/aip/general/0124/many_to_one.proto b/aip/general/0124/many_to_one.proto
new file mode 100644
index 00000000..598f03c9
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+++ b/aip/general/0124/many_to_one.proto
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+// Copyright 2021 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto3";
+
+import "google/api/resource.proto";
+
+// A representation of an individual person who writes a book.
+message Author {
+  option (google.api.resource) = {
+    type: "library.googleapis.com/Author"
+    pattern: "publishers/{publisher}/authors/{author}"
+  };
+
+  // The name of the author.
+  // Format: publishers/{publisher}/authors/{author}
+  string name = 1;
+
+  // The author's given name (in Western culture: "first name").
+  string given_name = 2;
+
+  // The author's family name (in Western culture: "last name").
+  string family_name = 3;
+}
+
+// A representation of a single book.
+message Book {
+  option (google.api.resource) = {
+    type: "library.googleapis.com/Book"
+    pattern: "publishers/{publisher}/books/{book}"
+  };
+
+  // The name of the book.
+  // Format: publishers/{publisher}/books/{book}
+  string name = 1;
+
+  // The resource name for the author of the book.
+  // Format: publishers/{publisher}/authors/{author}
+  string author = 2 [(google.api.resource_reference) = {
+    type: "library.googleapis.com/Author"
+  }];
+
+  // The rating assigned to the book.
+  float rating = 3;
+}
diff --git a/aip/general/0124/subresource.proto b/aip/general/0124/subresource.proto
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+++ b/aip/general/0124/subresource.proto
@@ -0,0 +1,73 @@
+// Copyright 2021 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto3";
+
+import "google/api/resource.proto";
+
+// A representation of an individual person who writes a book.
+message Author {
+  option (google.api.resource) = {
+    type: "library.googleapis.com/Author"
+    pattern: "publishers/{publisher}/authors/{author}"
+  };
+
+  // The name of the author.
+  // Format: publishers/{publisher}/authors/{author}
+  string name = 1;
+
+  // The author's given name (in Western culture: "first name").
+  string given_name = 2;
+
+  // The author's family name (in Western culture: "last name").
+  string family_name = 3;
+}
+
+// A representation of a single book.
+message Book {
+  option (google.api.resource) = {
+    type: "library.googleapis.com/Book"
+    pattern: "publishers/{publisher}/books/{book}"
+  };
+
+  // The name of the book.
+  // Format: publishers/{publisher}/books/{book}
+  string name = 1;
+
+  // The rating assigned to the book.
+  float rating = 2;
+}
+
+// A representation of the link between a particular book and a
+// particular author.
+message BookAuthor {
+  // The resource pattern for BookAuthor indicates that Book is the
+  // canonical parent.
+  option (google.api.resource) = {
+    type: "library.googleapis.com/BookAuthor"
+    pattern: "publishers/{publisher}/books/{book}/authors/{book_author}"
+  };
+
+  // The resource name for the book-author association.
+  // Format: publishers/{publisher}/books/{book}/authors/{book_author}
+  string name = 1;
+
+  // The resource name for the author.
+  // Format: publishers/{publisher}/authors/{author}
+  string author = 2 [(google.api.resource_reference) = {
+    type: "library.googleapis.com/Author"
+  }];
+
+  // Other fields...
+}