AllAccepted answers
Eduard Lebedyuk · Dec 23, 2016

Here's an idea on how to do it without triggers altogether.

1. Set IsLeader property only in case member is a leader. So its 1 or NULL.

2. Add unique index on (Team, IsLeader). Unique index can have any number of NULL records.

3. If you try to add more than one leader, you'll get an error:

ERROR #5808: Key not unique: Utils.TeamMember:IsLeaderIndex:^Utils.TeamMemberI("IsLeaderIndex"," 1"," 1") [%SaveData+14^Utils.TeamMember.1:USER]

Sample code:

Class Utils.TeamMember Extends %Persistent
{

Property Team As %String;

Property Member As %String;

Property IsLeader(VALUELIST = ",1");

Index IsLeaderIndex On (Team, IsLeader) [ Unique ];

/// do ##class(Utils.TeamMember).Test()
ClassMethod Test(AddTwoLeaders = {$$$YES})
{
    do ..%KillExtent()
    write $System.Status.GetErrorText(..Add(1, "Alice"))
    write $System.Status.GetErrorText(..Add(1, "Bob"))
    write $System.Status.GetErrorText(..Add(1, "Clover"))
    write $System.Status.GetErrorText(..Add(1, "Dave", 1))
    if AddTwoLeaders {
        write $System.Status.GetErrorText(..Add(1, "Helen", 1))
    }
}

ClassMethod Add(Team, Member, IsLeader = "")
{
    set obj = ..%New()
    set obj.Team = Team
    set obj.Member = Member
    set obj.IsLeader = IsLeader
    quit obj.%Save()
}

}
Eduard Lebedyuk · Dec 7, 2016

On the other note, if you use REST on 2016.1+ you can enable CORS support in a more organised way:

Class cors.REST Extends %CSP.REST
{
/// This parameter influences the CORS support. The default is an empty string meaning 'not specified'.
/// If set to true (1) then CORS processing is ON. If set to false (0) then CORS processing is OFF.
/// If left unset "" then the decision to process CORS is delegated to the setting on the URL map route.
Parameter HandleCorsRequest;    

XData UrlMap [ XMLNamespace = "http://www.intersystems.com/urlmap" ]
{
<Routes>
<Route Url="/:test" Method="GET" Call="Test" Cors="true"/>
</Routes>
}

/// This is the CORS request handler. User should override this method in their subclass
/// if they don't want the default behavior
ClassMethod OnHandleCorsRequest(pUrl As %String) As %Status
{

    #; The default implementation is simply to dispatch to the
    #; default handler
    Quit ..HandleDefaultCorsRequest(pUrl)
}

}

Note, that if you're okay with HandleDefaultCorsRequest then you don't need to redefine  OnHandleCorsRequest method and need supply only HandleCorsRequest or Cors attribute on per path basis.

Eduard Lebedyuk · Dec 7, 2016

In some cases Access-Control-Allow-Origin: * is not a valid value, so I usually determine origin host and supply it:

Do %response.SetHeader("Access-Control-Allow-Origin",..GetOrigins())

And GetOrigins method:

/// Get Origin from %request object
ClassMethod GetOrigins() As %String
{
    set url = %request.GetCgiEnv("HTTP_REFERER")
    return $p(url,"/",1,3) // get http(s)://origin.com:port
}

Also, sometimes additional headers may be required, here's one of the more permissive sets:

Do %response.SetHeader("Access-Control-Allow-Origin",..GetOrigins())
Do %response.SetHeader("Access-Control-Allow-Credentials","true")
Do %response.SetHeader("Access-Control-Allow-Methods","GET, PUT, POST, DELETE, OPTIONS")
Do %response.SetHeader("Access-Control-Max-Age","10000")
Do %response.SetHeader("Access-Control-Allow-Headers","Content-Type, Authorization, Accept-Language, X-Requested-With")