Insert list of children(Orders_OMS__c) to either relationship (account.Order_OMS__r or account.Order_OMS1__r)

Question

Have one custom object Orders_OMS__c.

Account has two child relationship with it

account.Order_OMS__r

account.Order_OMS1__r

Question:

How to insert list of newly created Orders_OMS__c into

account.Order_OMS__r

or

account.Order_OMS1__r

Below is the code [It always inserts only to one relation account.Order_OMS1__r and not to the other] Note: There are records with orderType.isOMSOrder==true and orderType.isOMSOrder==false

    insert newOmsOrders;
    if (orderType.isOMSOrder) {
        for (Orders_OMS__c omsOrder : newOmsOrders) {
            System.debug('adding to account.Order_OMS__r omsOrder.Id ' + omsOrder.Id);
            account.Order_OMS__r.add(omsOrder);
        }
        //account.Order_OMS__r.addall(newOmsOrders);
        insert account.Order_OMS__r;
        //upsert account.Order_OMS__r;
    } else {
        for (Orders_OMS__c omsOrder : newOmsOrders) {
            System.debug('adding to account.Order_OMS1__r omsOrder.Id ' + omsOrder.Id);
            account.Order_OMS1__r.add(omsOrder);
        }
        //account.Order_OMS1__r.addall(newOmsOrders);
        insert account.Order_OMS1__r;
        //upsert account.Order_OMS1__r;
    }

Answer 1

Unless you are working with the special case of external ids, the only way to create an association between two objects is using the id fields (that end __c in custom objects) not the relationship fields (that end in __r in custom objects).

The direction of the relationship is not entirely clear to me (as your relationship names are singular and it is helpful to use plurals for the "many" relationship name), but taking my best guess from your code setting the ids would look like this:

for (Orders_OMS__c omsOrder : newOmsOrders) {
    if (orderType.isOMSOrder) {
        // Corresponds to Order_OMS__r; may have a different name but will end __c
        omsOrder.Account__c = account.Id;
    } else {
        // Correspids to Order_OMS1__r; may have a different name but will end __c
        omsOrder.Account1__c = account.Id;
    }
}
insert newOmsOrders;

I can only assume that you are seeing one of the relationships being setup because of some code elsewhere in your code base.

I also suggest you:

System.debug('isOMSOrder=' + orderType.isOMSOrder);

to confirm that there are both true and false values.