Hello there! we have implemented very similar approach. i.e., reading a canned JSON from a file then replacing JSON objects from Data-source value.
Data-driven test case structure would look like this:
Groovy script would read JSON file and then replace the JSON objects and build new JSON payload for your REST request.
import net.sf.*
import net.sf.json.*
import net.sf.json.groovy.*
import groovy.json.JsonSlurper
import groovy.json.JsonBuilder
//## Get Directories ##//
def RequestPath = "C://TestUser//Request//"
log.info "Read canned JSON payload from $RequestPath"
log.info "Parsing JSON payload..."
//## READ canned request from a file to modify ##//
def JsonPayload = new File(RequestPath + "SampleJSON.txt").text
def JSONReq = new JsonSlurper().parseText JsonPayload
//## Get data from DataSource ##//
def userName = context.expand( '${DataSource#UserName}' )
def Password = context.expand( '${DataSource#Password}' )
log.info "Updating json nodes..."
JSONReq.userName = "$userName"
JSONReq.password = "$Password"
log.info "Updating JSON Payload at testCase level property eSignJSON"
//## build json body and write to a file for reference ##//
def REQUEST = new JsonBuilder(JSONReq).toString()
testRunner.testCase.setPropertyValue("JSONPayload", REQUEST)
log.info "JSON payload is $REQUEST"
In the Rest request point "JSONPayload" property to get the latest payload.
i..e., ${#TestCase#JSONPayload}
Hope this helps!
