Fix SQL container throughput update to preserve existing throughput buckets

src/cosmosdb-preview/azext_cosmosdb_preview/custom.py

@@ -2319,6 +2319,14 @@ def cli_cosmosdb_sql_container_throughput_update(client,
                                                  max_throughput=None,
                                                  throughput_buckets=None):
     """Update an Azure Cosmos DB SQL container throughput"""
+
+    # If throughput_buckets is None (not explicitly provided), preserve existing throughput buckets
+    if throughput_buckets is None:
+        logger.debug('reading SQL container throughput to preserve existing throughput buckets')
+        current_throughput = client.get_sql_container_throughput(resource_group_name, account_name, database_name, container_name)
+        if current_throughput.resource and current_throughput.resource.throughput_buckets is not None:
+            throughput_buckets = current_throughput.resource.throughput_buckets
+
     throughput_update_resource = _get_throughput_settings_update_parameters(throughput=throughput,
                                                                             max_throughput=max_throughput,
                                                                             throughput_buckets=throughput_buckets)