Search parameter for a reference example

{
  "resourceType" : "SearchParameter",
  "id" : "example-reference",
  "extension" : [{
    "url" : "http://hl7.org/fhir/StructureDefinition/structuredefinition-wg",
    "valueCode" : "fhir"
  }],
  "url" : "http://hl7.org/fhir/SearchParameter/example-reference",
  "name" : "ExampleSearchParameter",
  "title" : "Example Search Parameter",
  "status" : "draft",
  "experimental" : true,
  "date" : "2013-10-23",
  "publisher" : "HL7 International / FHIR Infrastructure",
  "contact" : [{
    "telecom" : [{
      "system" : "url",
      "value" : "http://www.hl7.org/Special/committees/fiwg"
    }]
  }],
  "description" : "Search by condition subject",
  "purpose" : "Need to search Condition by subject",
  "code" : "subject",
  "base" : ["Condition"],
  "type" : "reference",
  "expression" : "Condition.subject",
  "processingMode" : "normal",
  "target" : ["Organization"],
  "modifier" : ["missing"],
  "chain" : ["name",
  "identifier"]
}