如何为 JSON 对象中的某个属性创建索引

内容摘自英文社区，原文见这里。主要解答了如何对于JSON对象中的某个字段创建索引。

在解答中Robert Cemper给出了下面示例

主要思路是将%Stream --> %DynamicObject --> 提取索引字段内容保存到计算字段内

Property JSON As %Stream.GlobalCharacter;

Property FirstName As %String [ Calculated, SqlComputed , SqlComputeCode = { set {*}=..GetDyn({ID},"FirstName") } ];

Property LastName As %String [ Calculated, SqlComputed , SqlComputeCode = { set {*}=..GetDyn({ID},"LastName") } ];

Index fn on FirstName; Index ln on LastName;

ClassMethod GetDyn(ID As %Integer, item = "") As %String
{    
  set JSON=..%OpenId(ID).JSON    
  do JSON.Rewind()    
  set st=JSON.Read(3000000)    
  set dyno={}.%FromJSON(st)    
  set rep=$Property(dyno,item)    
  quit rep
}

同时 Eduard Lebedyuk 给出了另一种写法，注意其中JSON 保存为%Library.DynamicAbstractObject

/// This property holds the document state. Serialization is JSON. 
Property JSON As %Library.DynamicAbstractObject;

/// This is an automatically calculated property "$.firstName"
Property firstName As %VarString [ SqlComputeCode = { set {*}=$$%EvaluatePathOne^%DocDB.Document({JSON},"$.firstName")}, SqlComputed, SqlComputeOnChange = %Doc ];

/// Index on firstName property
Index firstName On firstName;