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santhosh konathala 8santhosh konathala 8 

How can we achieve only one child to one parent using a trigger

Dayakar.DDayakar.D
Hi sonthosh,

Here I am taking an example of account and contact as a parent and child.
and below is the trigger code to stop inserting more than one child to a parent 
trigger OnlyOnechild on Contact (before insert) {
    public set<id> accountIds=new set<id>();
    for(Contact contactVar:trigger.new)
    {
        accountIds.add(contactVar.AccountId);
    }
    list<Contact> contactList=[select id,name from Contact where AccountId in :accountIds];
    for(Contact conVar:trigger.new)
    {
        if(contactList.size()>=1)
        	conVar.addError('Only one child is allowed ');
    }
    

}

If it solves your problem, please let me know.
Best Regards,
Dayakar
 
JyothsnaJyothsna (Salesforce Developers) 
Hi Santhosh,

You can achecive this functionality by using one-to-one relationship.

Let's consider the scenario that we would like to establish a One-to-One relationship between Accounts and Contacts.

Step 1:

create a master-detail relationship and than on master create a roll-up summary field of child with count. then you write validation on rule rollup summary field to check for >1 .
 so it will give you error if it has more than one record for same master detail relation values.

Step 2:

i. Create a lookup field on the child
ii. Create a unique field on the child, and hide this field from all page layouts
iii. Write a workflow, for any change of the lookup field, to copy that value from the lookup field into the unique field

Done!! you have established a one to one relationship between Account and Contacts...

When you try to add a second contact to the Account, the "unique" constraint would be violated and an error would be thrown.

Please check the below link for related posts.
https://developer.salesforce.com/forums/?id=906F0000000AfsKIAS

Hope this helps you!
Best Regards,
Jyothsna