Odd behavior with using transform function to postprocess AST in order to rename json field name

I think all you're missing is backticks around oldFieldName:

scala> def renameFields(originalJson : JValue, oldFieldName : String, newFieldName : String): JValue = originalJson transform { case JField(`oldFieldName`,x) => JField(newFieldName, x)}renameFields: (originalJson: net.liftweb.json.JsonAST.JValue,oldFieldName: String,newFieldName: String)net.liftweb.json.JsonAST.JValuescala> val jstring = """ { "aisle" : 1, "bin" : 1, "hasWhat" : [{ "id" : 4, "name" : "Granny", "color" : "green"}, { "id" : 4, "name" : "Fuji", "color" : "red"}] }"""jstring: java.lang.String =  { "aisle" : 1, "bin" : 1, "hasWhat" : [{ "id" : 4, "name" : "Granny", "color" : "green"}, { "id" : 4, "name" : "Fuji", "color" : "red"}] }scala> val json = parse(jstring)json: net.liftweb.json.JsonAST.JValue = JObject(List(JField(aisle,JInt(1)), JField(bin,JInt(1)), JField(hasWhat,JArray(List(JObject(List(JField(id,JInt(4)), JField(name,JString(Granny)), JField(color,JString(green)))), JObject(List(JField(id,JInt(4)), JField(name,JString(Fuji)), JField(color,JString(red)))))))))scala> renameFields(json,"aisle","row")res0: net.liftweb.json.JsonAST.JValue = JObject(List(JField(row,JInt(1)), JField(bin,JInt(1)), JField(hasWhat,JArray(List(JObject(List(JField(id,JInt(4)), JField(name,JString(Granny)), JField(color,JString(green)))), JObject(List(JField(id,JInt(4)), JField(name,JString(Fuji)), JField(color,JString(red)))))))))

Without the backticks, case JField(oldFieldName,x) is saying "bind whatever value I find as the JField name to the new variable oldFieldName" and thus your original oldFieldName variable is shadowed. With the backticks around oldFieldName, you're saying you only want to match a JField whose name is the value of oldFieldName.