Apex Trigger with custom objects - salesforce

Question

2 custom objects
1.Merchandise
2.invoice
whenever a new merchandise is created an auto invoice has to be created.As am new to apex,please bear with me.Any one please correct my code.
code:
trigger createinvoice on Merchandise2__c (after insert,after update) {
list<Invoice2__c>line = new list<Invoice2__c>();
for(Merchandise2__c mer:Trigger.new){
Invoice2__c li = new Invoice2__c();
line =[select id from Invoice2__c ];
li.name = mer.Name;
li.Status__c='open';
li.id = mer.id;
line.add(li);
}
insert line;
}

Answer 1

First remove the after update from your trigger unless you want a new invoice created each time someone updates the Merchandise record.
I'm not sure what you were trying to accomplish with the select line.
"line =[select id from Invoice__c];" this doesn't seem to accomplish anything. Also you're trying to set the id field of the invoice to the id of the merchandise record. You can't do that. You'll need a lookup field on the Invoice record that points to merchandise. Below I called it merchandiseKeyField.
Hope this helps.
trigger createinvoice on Merchandise2__c (after insert,after update) {
//if you don't remove after Update then check which trigger
if(Trigger.IsInsert){
list<Invoice2__c> invoices = new list<Invoice2__c>();
for(Merchandise__c mer: trigger.new){
invoices.add(new Invoice2__c(li.name = mer.name,
li.merchandiseKeyField = mer.id);
}
insert invoices;
}
}

Answer 2

replace your loop with this:
for(Merchandise2__c mer:Trigger.new){
Invoice2__c li = new Invoice2__c();
li.name = mer.Name;
li.Status__c='open';
line.add(li);
}