Dataweave equivalent of SQL join - mule

Question

How can I do the equivalent of a SQL join in Dataweave? I'm basically implementing a lookup. I have CSV data and JSON data in flow variables. I'm able to use both of them, but I can't figure out how connect them. Let's say the CSV has two columns:
Name,ExternalId
Foo,1
Bar,2
Baz,2
The JSON data is:
{
ExternalEntities: [
{ "Id": 1, "Name": "One", "Description": "Thing One" }
, { "Id": 2, "Name": "Two", "Description": "Thing Two" }
]
}
In the end, I'd like a List<Hashmap> with the following (expressed here in JSON format for convenience.)
[
{ "Name": "Foo", "ExternalName": "One", "ExternalDescription": "Thing One" }
, { "Name": "Bar", "ExternalName": "Two", "ExternalDescription": "Thing Two" }
, { "Name": "Baz", "ExternalName": "Two", "ExternalDescription": "Thing Two" }
]

Answer 1

So here is a sample using the json as the payload and the csv as a lookup table. The first flows parses the json and does a flow lookup passing the 'Id' as the payload. The second flow loads the csv as an array and does a search by the Id passed in. This returns a map of the CSV record found where you can then extract the 'Name' field.
It is possible to do this in one transformer, but for demo sake this was easier. You can also reverse the lookup with some tinkering if you want the csv as your payload and the json as your lookup.
<flow name="lookuptest" processingStrategy="synchronous">
<poll doc:name="Poll">
<logger level="INFO" doc:name="Logger" />
</poll>
<set-payload
value="{ "ExternalEntities": [ { "Id": 1, "Name": "One", "Description": "Thing One" } , { "Id": 2, "Name": "Two", "Description": "Thing Two" } ] }"
doc:name="Set Payload" mimeType="application/json"></set-payload>
<dw:transform-message doc:name="Transform Message">
<dw:input-payload doc:sample="string_2.dwl" />
<dw:set-payload><![CDATA[%dw 1.0
%input payload application/json
%output application/json
---
payload.ExternalEntities map ((value , index) -> {
Name: lookup("NameLookup", [value.Id as :string]).Name
})]]></dw:set-payload>
</dw:transform-message>
<object-to-string-transformer doc:name="Object to String"/>
<logger level="ERROR" message="#[payload]" doc:name="Logger"/>
</flow>
<flow name="NameLookup" processingStrategy="synchronous">
<set-variable value="#[payload[0]]" variableName="ExternalId"
doc:name="Variable" />
<set-payload
value="#[Thread.currentThread().getContextClassLoader().getResourceAsStream('lookuptables/namelookup.csv')]"
mimeType="application/csv" doc:name="Variable" />
<dw:transform-message doc:name="Transform Message">
<dw:input-payload doc:sample="string_2.dwl" />
<dw:set-payload><![CDATA[%dw 1.0
%input payload application/csv
%output application/java
---
(payload[?($.ExternalId == flowVars['ExternalId'])])[0]
]]></dw:set-payload>
</dw:transform-message>
</flow>

Answer 2

Here's a second solution that works, though I accepted #Ryan Carter's answer.
%dw 1.0
%output application/json
---
flowVars.myCsv map (
(row0, i) -> using (lookupElement = (flowVars.jsonLookup.ExternalEntities filter ((obj1) -> row0.ExternalId ~= obj1.Id))[0]) {
Name: row0.Name
, ExternalName: lookupElement.Name
, ExternalDescription: lookupElement.Description
} mapObject ({"$$": $, (StartsWithB:$[0] == "B") when $$ ~= "Name"})
)