A brief post about async postbacks from javascript

Just in case you want to cause an async postback from javascript its easiest to use  ClientScriptManager.GetPostbackRef with client option set and put an invisible link button somewhere.

Programmatically populating a BDC field in Sharepoint 2010

In this post I try to explain how to retrieve and store data in a external lookup column or business data field. For the code snippets I use PowerShell instead of C# because I can test my claims as I write.

Lets set up some pseudo vars

$list=$web.Lists["ListWithExternalRef"]

$listItem=$list[0]

$bdcField=$list.Fields["ExternalItemField"]

$listItem["bdcColName"] holds the value of the BDC field which was specified when setting up the column, which can be set in SPBusinessDataField.BdcFieldName. The data is always stored as a string.

The BCS Identity of the item is stored in $listItem[$bdcField.RelatedField]. 

(Still need to check if this is EntityInstanceReference or Identity
Check BCSFieldEditor and ItemPicker with .NET Reflector )

If you want to deserialize the EntityInstanceReference you need to get an instance to the matching IMetadataCatalog :
$bdcService=(Get-SPService).services|?{$_.typename -eq "Business Data Connectivity Service"}

$bdcCat=$bdcService.GetDatabaseBackedMetadataCatalog((Get-SPServiceContext http://yourdevsite))

[Microsoft.BusinessData.Runtime.entityinstancereference]::Deserialize($bcsRef,$bdcCat)

For each additional external column which is stored with this field Sharepoint creates a field in the list. The additional fields names can be obtained with $bdcField.GetSecondaryFieldsNames(). To get a reference to the field you can use the display name which is in the  format "$($bdcField.Title): $secondaryFieldName". 

Sharepoint doesnt set the secondary lookup values itself when you save the entry, so you have to retrieve all values for the secondary field names.