Implemented the GET /users/{username}/collection/folders/{folder_id}/releases/{release_id}/instances/{instance_id}/fields/{field_id} endpoint for the Open API specification document in the library target.

Sources/DiscogsService/openapi.yaml

@@ -1453,6 +1453,40 @@ paths:
           $ref: '#/components/responses/NotFound'
         '500':
           $ref: '#/components/responses/InternalError'
+  /users/{username}/collection/folders/{folder_id}/releases/{release_id}/instances/{instance_id}/fields/{field_id}:
+    post:
+      tags: 
+        - User Collection
+      summary: Update a value of a custom field assigned to a release instance.
+      description: Changes the value of a notes field on a particular instance.
+      operationId: editFieldsInstance
+      parameters:
+        - $ref: '#/components/parameters/Username'
+        - $ref: '#/components/parameters/FolderId'
+        - $ref: '#/components/parameters/ReleaseId'
+        - $ref: '#/components/parameters/InstanceId'
+        - $ref: '#/components/parameters/FieldId'
+        - $ref: '#/components/parameters/FieldValue'
+      security:
+        - ConsumerKeySecret: []
+        - UserToken: []
+        - OAuth: []
+      responses:
+        '204':
+          description: Successfully updated a value of a custom notes field for a release instance in a collection folder.
+          headers:
+            X-Discogs-RateLimit:
+              $ref: '#/components/headers/RateLimit'
+            X-Discogs-RateLimit-Used:
+              $ref: '#/components/headers/RateLimitUsed'
+            X-Discogs-RateLimit-Remaining:
+              $ref: '#/components/headers/RateLimitRemaining'
+        '403':
+          $ref: '#/components/responses/NotAccessable'
+        '404':
+          $ref: '#/components/responses/NotFound'
+        '500':
+          $ref: '#/components/responses/InternalError'
   /users/{username}/collection/fields:
     get:
       tags:
@@ -1598,6 +1632,22 @@ components:
       in: query
       schema:
         $ref: '#/components/schemas/Currency'
+    FieldId:
+      description: An identifier of a custom field.
+      name: field_id
+      in: path
+      required: true
+      schema:
+        type: integer
+        minimum: 1
+      example: 8
+    FieldValue:
+      description: A value of a custom field.
+      name: value
+      in: query
+      required: true
+      schema:
+        type: string
     FolderId:
       description: An identifier of a collection folder.
       name: folder_id