Multiple entities selection in single look up:

If you create custom lookup in CRM, this lookup is oriented only for one entity.
I want to add possibility to select from some entities in one lookup.

I have custom lookup "originatingleadid". This lookup is oriented on Lead entity. I want to select from views of three different entities: Lead, Contact, User.

For this I've done some customization in opportunity entity, I've created three hidden text fields:
new_type - to save lookup type
new_guid - to save lookup guid
new_name - to save lookup name.
That is all customization.

I've developed three javascript function:

1. function to handle OnChange event on lookup.

function onchange()
{
var originatingLeadValue = Xrm.Page.getAttribute("originatingleadid").getValue();
if (IsNull(originatingLeadValue) || originatingLeadValue[0].type == "4")
{
// Lookup is null or there is a Lead, need to clear hidden fields. To clear one of them will be enough, we will be check this field during onload.
Xrm.Page.getAttribute("new_guid").setValue(null);
}
else
{
// Lookup contains Contact or User, so need to store lookup properties to our hidden fields.
Xrm.Page.getAttribute("new_type").setValue(originatingLeadValue[0].type);
Xrm.Page.getAttribute("new_guid").setValue(originatingLeadValue[0].id);
Xrm.Page.getAttribute("new_name").setValue(originatingLeadValue[0].name);
}
}

2. function to handle OnSave event on Opportuniry Form.

function onsave()
{
var originatingLeadValue = Xrm.Page.getAttribute("originatingleadid").getValue();
if (!IsNull(originatingLeadValue) && originatingLeadValue[0].type != '4')
{
Xrm.Page.getAttribute("originatingleadid").setValue(null);
}
}

 3. function to handle OnLoad event on Opportunity Form.

function onload()
{
lookuptypeIcons = '/_imgs/ico_16_2.gif:/_imgs/ico_16_4.gif:/_imgs/ico_16_8.gif';
lookuptypenames = 'contact:2:Contact,lead:4:Lead,systemuser:8:User';
lookuptypes = '2,4,8';
var savedId = Xrm.Page.getAttribute("new_guid").getValue();
var savedType = Xrm.Page.getAttribute("new_type").getValue();
var savedName = Xrm.Page.getAttribute("new_name").getValue();
var savedEntityName = savedType == "8" ? "systemuser" : "contact";
document.getElementById("originatingleadid").setAttribute("lookuptypes", lookuptypes);
document.getElementById("originatingleadid").setAttribute("lookuptypenames", lookuptypenames);
document.getElementById("originatingleadid").setAttribute("lookuptypeIcons", lookuptypeIcons);
document.getElementById("originatingleadid").setAttribute("defaulttype", "4"); // default type - Lead entity
var originatingLeadValue = Xrm.Page.getAttribute("originatingleadid").getValue();
if (IsNull(originatingLeadValue) && !IsNull(savedId))
{
var value = new Array();
value[0] = new Object();
value[0].displayClass = "ms-crm-Lookup-Item";
value[0].keyValues = new Object();
value[0].values = new Object();
value[0].onclick = "openlui()";
value[0].id = savedId;
value[0].entityType = savedEntityName;
value[0].typename = savedEntityName;
value[0].name = savedName;
value[0].type = savedType;
Xrm.Page.getAttribute("originatingleadid").setValue(value);
}
}

So, now you can select from  three different entity in one lookup :-)

Custom Lookup view