how to define relate field lookup value working in the modules

Question

I took the following sample off Cases module and tried to apply same pattern but it is not working for me

'site_mxxn' => array( 'name' => 'site_mxxn', 'rname' => 'name', 'id_name' => 'site_id', 'vname' => 'LBL_SITE_MXXN', 'type' => 'relate', 'related_fields' => array( 'site_id', ), 'link' => 'accounts_workorder_1', 'table' => 'accounts_cstm', //--> THE COLUMN I NEED TO DISPLAY ON FRONTEND IS ON THIS TABLE CALLED MXXN 'join_name' => 'accounts', 'isnull' => 'true', 'module' => 'CustomAccount', 'dbType' => 'varchar', 'len' => 100, 'source' => 'non-db', 'unified_search' => true, 'comment' => 'The name of the site represented by the site_id field', 'required' => true, 'importable' => 'required', 'exportable' => true, 'studio' => array( 'portalrecordview' => false, 'portallistview' => false, ), ), 'site_id' => array( 'name' => 'site_id', 'type' => 'relate', 'dbType' => 'id', 'rname' => 'id', 'module' => 'CustomAccounts', 'id_name' => 'site_id', 'reportable' => false, 'vname' => 'LBL_SITE_ID', 'audited' => true, 'massupdate' => false, 'comment' => 'The site to which the order is associated', ), I keep getting an error when I try to save the record PHP Fatal error: Uncaught Error: Call to a member function getOwnerField() on null in /var/www/html/include/SugarQuery/Builder/Field/Select.php:117
Stack trace:
#0 /var/www/html/include/SugarQuery/Builder/Field.php(112): SugarQuery_Builder_Field_Select->expandField()
#1 /var/www/html/include/SugarQuery/Builder/Field.php(87): SugarQuery_Builder_Field->setupField(Object(SugarQuery))
#2 /var/www/html/include/SugarQuery/Builder/Field/Select.php(23): SugarQuery_Builder_Field->__construct('site_mxxn', Object(SugarQuery))
#3 /var/www/html/include/SugarQuery/Builder/Select.php(53): SugarQuery_Builder_Field_Select->__construct('site_mxxn', Object(SugarQuery))
#4 /var/www/html/include/SugarQuery/SugarQuery.php(205): SugarQuery_Builder_Select->field(Array)
#5 /var/www/html/data/SugarBean.php(3505): SugarQuery->select(Array)
#6 /var/www/html/clients/base/api/FilterApi.php(712): SugarBean->fetchFromQuery(Object(SugarQuery), Array, Array)
#7 /var/www/html/clients/base/api/FilterApi.php(445): FilterApi->runQuery(Object(RestService), Array, in /var/www/html/include/SugarQuery/Builder/Field/Select.php on line 117, referer: http://localhost/

I have 2 custom modules, ORDER 1:->M SITE 
 I want to create a lookup field on ORDER related to SITE so when on record view for order I want to type in site name and be able to select it from dropdown list and save in db for ORDER 
 
 SITE extends the accounts module

a site can have many orders

sharif uddin · Answer

Is there a reason you are trying to do this in code? 
 It sounds like you have two modules: Orders and Sites. 
 You want to have a relate field on Order to link it to a Site. 
 You can do this two ways and you can do it via Studio: 
 Relate Field (which adds a foreign key to your Orders table for the ID of the Site) 
 1:M Relationship (which adds a Relationship table to your database with foreign keys for both the Orders and the Site) 
 There are differences between the two, there are good reasons for each one, personally I prefer Relationships because I find them easier to work with when manipulating data, customizing subpanel actions etc. 
 Try reviewing this, it should help: https://support.sugarcrm.com/Knowledge_Base/Studio_and_Module_Builder/Introduction_to_Relationships_and_Relate_Fields/ 
 Francesca