fix(contracts): Add conditional check for contract status updates

Author: sliedig

Unicorn.Contracts/ContractsService/ContractEventHandler.cs

@@ -205,15 +205,18 @@ private async Task UpdateContractAsync(SQSEvent.SQSMessage sqsMessage)
                     { "PropertyId", new AttributeValue { S = updateContractRequest.PropertyId } }
                 },
                 UpdateExpression = "SET #contract_status=:cs, #modified_date=:md",
+                ConditionExpression = "attribute_exists(#property_id) AND #contract_status = :draft_status",
                 ExpressionAttributeNames = new Dictionary<string, string>
                 {
+                    { "#property_id", "PropertyId" },
                     { "#modified_date", "ContractLastModifiedOn" },
                     { "#contract_status", "ContractStatus" }
                 },
                 ExpressionAttributeValues = new Dictionary<string, AttributeValue>
                 {
                     { ":md", new AttributeValue { S = DateTime.Now.ToString("O") } },
-                    { ":cs", new AttributeValue { S = ContractStatus.Approved } }
+                    { ":cs", new AttributeValue { S = ContractStatus.Approved } },
+                    { ":draft_status", new AttributeValue { S = ContractStatus.Draft } }
                 },
                 ReturnValues = "UPDATED_NEW"
             };
@@ -222,6 +225,10 @@ private async Task UpdateContractAsync(SQSEvent.SQSMessage sqsMessage)
 
             Logger.LogInformation(response);
         }
+        catch (ConditionalCheckFailedException e)
+        {
+            Logger.LogError("Unable to update contract, because `{e.Message}`. Perhaps you are trying to add a contract that already has an active status?", e.Message);
+        }
         catch (Exception e)
         {
             Logger.LogError(e.Message);