Retrive Single Record Data using OData

In order to retrieve data of a record  using OData. Let us take a scenario in which I have a  A,B and Test entities, A entity form has lookup of B entity and Test entity has lookup of both A and B entity. So I have created some records in B entity and A entity. Now In my Test Entity form when I select a Lookup of A, The lookup in B must Automatically selected. Since A entity form has lookup of B entity, i.e, In A-record form, B-record is selected in lookup.

So the above snippet code can be used on change event of a A-lookup.

function customentity_onchange() {

debugger;
    var lookupObject = Xrm.Page.getAttribute("schema-name-of-Alookup").getValue();
    var z=lookupObject[0].id; // Guid of lookup
    alert(z);
    if (z != null) {
        var entityA="A-entitynameSet";  //A-entity name with "Set" attached to it for Odata Query.
        var field = "B-lookup_feild_name_in_A-Entity";   //lookup feild name in A entity.
        var a = GetAdata(z,entityA,field);
        if(a!=null)           
        {
        //alert(a[0]["B-lookup_feild_name_in_A-Entity"].Name);
        var lookupData = new Array();
        var lookupItem = new Object();
        lookupItem.id=a[0]["B-lookup_feild_name_in_A-Entity"].Id;
        alert(lookupItem.id);
        lookupItem.name = a[0]["B-lookup_feild_name_in_A-Entity"].Name;
        alert(lookupItem.name);
        lookupItem.entityType ="b-Entity_name";
        alert(lookupItem.entityType);
        lookupData[0] = lookupItem;
        Xrm.Page.getAttribute("schemaname_of_b_lookup_in_TestEntity").setValue(lookupData);
        //Xrm.Page.getAttribute("hrm_b").setValue(a);
        }
        }
    }
function GetAdata(z,entityA,field) {
    var serverUrl = Xrm.Page.context.getServerUrl();
    var oDataUri = serverUrl + "/XRMServices/2011/OrganizationData.svc/"+entityA+"(guid'" + z + "')?$select="+field;
    var jSonArray = new Array();
    $.ajax({
        type: "GET",
        contentType: "application/json; charset=utf-8",
        datatype: "json",
        url: oDataUri,
        async: false,
        beforeSend: function (XMLHttpRequest) {
            //Specifying this header ensures that the results will be returned as JSON.           
            XMLHttpRequest.setRequestHeader("Accept", "application/json");
            //alert(hai);
        },
        success: function (data, textStatus, XmlHttpRequest) {
            if (data && data.d != null) {
                jSonArray.push(data.d);
            }
        },
        error: function (XmlHttpRequest, textStatus, errorThrown) {
        alert("Status: " + textStatus + "; ErrorThrown: " + errorThrown);
            //alert("Error :  has occured during retrieval of the A details");
    }
    });

    return jSonArray;
}