fix(rds): proxy for db cluster fails with model validation error (#8896)

fixes #8885
follows #8476

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-rds/lib/proxy.ts

@@ -408,20 +408,30 @@ export class DatabaseProxy extends cdk.Resource
     this.endpoint = this.resource.attrEndpoint;
 
     let dbInstanceIdentifiers: string[] | undefined;
-    if (bindResult.dbClusters) {
-      // support for only instances of a single cluster
-      dbInstanceIdentifiers = bindResult.dbClusters[0].instanceIdentifiers;
-    } else if (bindResult.dbInstances) {
+    if (bindResult.dbInstances) {
       // support for only single instance
       dbInstanceIdentifiers = [ bindResult.dbInstances[0].instanceIdentifier ];
     }
 
-    new CfnDBProxyTargetGroup(this, 'ProxyTargetGroup', {
+    let dbClusterIdentifiers: string[] | undefined;
+    if (bindResult.dbClusters) {
+      dbClusterIdentifiers = bindResult.dbClusters.map((c) => c.clusterIdentifier);
+    }
+
+    if (!!dbInstanceIdentifiers && !!dbClusterIdentifiers) {
+      throw new Error('Cannot specify both dbInstanceIdentifiers and dbClusterIdentifiers');
+    }
+
+    const proxyTargetGroup = new CfnDBProxyTargetGroup(this, 'ProxyTargetGroup', {
       dbProxyName: this.dbProxyName,
       dbInstanceIdentifiers,
-      dbClusterIdentifiers: bindResult.dbClusters?.map((c) => c.clusterIdentifier),
+      dbClusterIdentifiers,
       connectionPoolConfigurationInfo: toConnectionPoolConfigurationInfo(props),
     });
+
+    // Currently(2020-07-04), this property must be set to default.
+    // https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-rds-dbproxytargetgroup.html#TargetGroupName-fn::getatt
+    proxyTargetGroup.addOverride('Properties.TargetGroupName', 'default');
   }
 
   /**

packages/@aws-cdk/aws-rds/test/integ.proxy.expected.json

@@ -554,7 +554,8 @@
           {
             "Ref": "dbInstance4076B1EC"
           }
-        ]
+        ],
+        "TargetGroupName": "default"
       }
     }
   }

packages/@aws-cdk/aws-rds/test/test.proxy.ts

@@ -1,4 +1,4 @@
-import { expect, haveResource, ResourcePart } from '@aws-cdk/assert';
+import { ABSENT, expect, haveResource, ResourcePart } from '@aws-cdk/assert';
 import * as ec2 from '@aws-cdk/aws-ec2';
 import * as cdk from '@aws-cdk/core';
 import { Test } from 'nodeunit';
@@ -67,6 +67,7 @@ export = {
             Ref: 'InstanceC1063A87',
           },
         ],
+        TargetGroupName: 'default',
       },
     }, ResourcePart.CompleteDefinition));
 
@@ -140,17 +141,42 @@ export = {
             Ref: 'DatabaseB269D8BB',
           },
         ],
-        DBInstanceIdentifiers: [
-          {
-            Ref: 'DatabaseInstance1844F58FD',
-          },
-          {
-            Ref: 'DatabaseInstance2AA380DEE',
-          },
-        ],
+        TargetGroupName: 'default',
       },
     }, ResourcePart.CompleteDefinition));
 
     test.done();
   },
+
+  'Cannot specify both dbInstanceIdentifiers and dbClusterIdentifiers'(test: Test) {
+    // GIVEN
+    const stack = new cdk.Stack();
+    const vpc = new ec2.Vpc(stack, 'VPC');
+    const cluster = new rds.DatabaseCluster(stack, 'Database', {
+      engine: rds.DatabaseClusterEngine.AURORA_POSTGRESQL,
+      engineVersion: '10.7',
+      masterUser: {
+        username: 'admin',
+      },
+      instanceProps: {
+        instanceType: ec2.InstanceType.of(ec2.InstanceClass.BURSTABLE2, ec2.InstanceSize.SMALL),
+        vpc,
+      },
+    });
+
+    // WHEN
+    test.doesNotThrow(() => {
+      new rds.DatabaseProxy(stack, 'Proxy', {
+        proxyTarget: rds.ProxyTarget.fromCluster(cluster),
+        secret: cluster.secret!,
+        vpc,
+      });
+    }, /Cannot specify both dbInstanceIdentifiers and dbClusterIdentifiers/);
+
+    expect(stack).to(haveResource('AWS::RDS::DBProxyTargetGroup', {
+      DBInstanceIdentifiers: ABSENT,
+    }, ResourcePart.Properties));
+
+    test.done();
+  },
 };