Scatter-Gather: merge into one map - mule

Question

I need to query several tables and at the end of the scatter-gather flow I want to have a payload of this form
{table_1: [ list of results ], table_2: [ list of results ]}
I tried this:
<scatter-gather doc:name="Scatter-Gather">
<processor-chain>
<db:select config-ref="dataSrouce" doc:name="Table 1">
<db:parameterized-query><![CDATA[// my query
</db:select>
<set-payload value="#[['table_1': #[payload]]]" doc:name="Set Payload"/>
</processor-chain>
<processor-chain>
<db:select config-ref="dataSrouce" doc:name="Table 2">
<db:parameterized-query><![CDATA[// my query
</db:select>
<set-payload value="#[['table_2': #[payload]]]" doc:name="Set Payload"/>
</processor-chain>
</scatter-gather>
<set-payload value="#[groovy:payload.inject([:]) {result, entry -> result += entry; result}]" doc:name="Set Payload"/>
But getting an exception:
Exception was found for route(s): 1. Message payload is of type: CaseInsensitiveHashMap (org.mule.routing.CompositeRouting
Exception)
org.mule.routing.CollectAllAggregationStrategy:51 (http://www.mulesoft.org/docs/site/current3/apidocs/org/mule/routing/Comp
ositeRoutingException.html)

Answer 1

In each branch of your Scatter-Gather place an ObjectToString and an ObjectToJSON Transformer.
Then after the Scatter-Gather place a SetPayload with the following value:
{
"table_1": #[payload[0]],
"table_2": #[payload[1]]
}
Example:
<scatter-gather doc:name="Scatter-Gather">
<processor-chain>
<flow-ref name="Subflow_1" doc:name="Subflow_1"/>
<object-to-string-transformer doc:name="Object to String"/>
<json:object-to-json-transformer doc:name="Object to JSON"/>
</processor-chain>
<processor-chain>
<flow-ref name="Subflow_2" doc:name="Subflow_2"/>
<object-to-string-transformer doc:name="Object to String"/>
<json:object-to-json-transformer doc:name="Object to JSON"/>
</processor-chain>
</scatter-gather>
<set-payload value="{
"table_1": #[payload[0]],
"table_2": #[payload[1]]}"
doc:name="Set Payload" mimeType="application/json"/>