Salesforce Trigger Test Class - triggers

Question

below is my Apex Trigger. I am a beginner and trying to write its test class but continuously getting error "System.DmlException: Insert failed. First exception on row 0; first error: REQUIRED_FIELD_MISSING, Error: You can't select products until you've chosen a price book for this opportunity on the products related list.: []".
trigger TrgrOptyHighestCustmorePrice on Opportunity (before insert, before update)
{
public Id oid;
public String bidType;
public String BUCode;
for(Opportunity o : trigger.new)
{
oid = o.Id;
bidType = o.BidType__c;
BUCode = o.Business_Line_BU__c;
}
List<OpportunityLineItem> oliList = new list<OpportunityLineItem>([SELECT id, Customer_Price__c, ReCat_Product_Line__c
FROM OpportunityLineItem
WHERE OpportunityId =: oid ORDER BY
Customer_Price__c DESC LIMIT 1]);
for(OpportunityLineItem oli : oliList)
{
if(bidType == 'Competitive' && oli.ReCat_Product_Line__c == 'DMS')
{
BUCode = 'BL.619';
}
if(bidType == 'Competitive' && (oli.ReCat_Product_Line__c == 'EMS' || oli.ReCat_Product_Line__c == 'GMS'))
{
BUCode = 'BL.620';
}
if(bidType == 'Competitive' && oli.ReCat_Product_Line__c == 'MMS')
{
BUCode = 'BL.622';
}
if(bidType == 'Sole Sourced' && oli.ReCat_Product_Line__c == 'DMS')
{
BUCode = 'BL.624';
}
if(bidType == 'Sole Sourced' && (oli.ReCat_Product_Line__c == 'EMS' || oli.ReCat_Product_Line__c == 'GMS'))
{
BUCode = 'BL.621';
}
if(bidType == 'Sole Sourced' && oli.ReCat_Product_Line__c == 'MMS')
{
BUCode = 'BL.623';
}
}
for(Opportunity opt : trigger.new)
{
opt.Business_Line_BU__c = BUCode;
}
}
Test Class
#isTest(seeAllData=true)
public class Test_TrgrOptyHighestCustmorePrice {
private static testmethod void TrgrOptyHighestCustmorePriceTest(){
Test.startTest();
//Insert a test product.
Product2 p1 = new Product2(Name='Product Monthly 1111', isActive=true, CurrencyIsoCode='USD', ReCat_Product_Line__c = 'DMS');
insert p1;
// Get standard price book ID.
Id pricebookId = Test.getStandardPricebookId();
// Insert a price book entry for the standard price book.
PricebookEntry standardPrice = new PricebookEntry(
Pricebook2Id = pricebookId, Product2Id = p1.Id,
UnitPrice = 10000, IsActive = true);
insert standardPrice;
Pricebook2 customPB = new Pricebook2(Name='Custom Pricebook', isActive=true);
insert customPB;
PricebookEntry customPrice = new PricebookEntry(
Pricebook2Id = customPB.Id, Product2Id = p1.Id,
UnitPrice = 12000, IsActive = true);
insert customPrice;
// Insert Opportunity
Opportunity opt = new Opportunity(Name='Test',StageName='Prospect',
CloseDate=date.today(),BidType__c = 'Competitive',
Business_Line_BU__c = 'BL.619',
PriceBook2 = customPB);
insert opt;
OpportunityLineItem optLI = new OpportunityLineItem(OpportunityId = opt.id, Product2Id = p1.Id);
insert optLI;
update opt;
Test.stopTest();
}
}
I am unable to understand how can I test my simple trigger.

Answer 1

Its because u do not fill all required fields for the Opportunity Line Item. See: https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_objects_opportunitylineitem.htm for required fields.
This as an example will work:
OpportunityLineItem optLI = new OpportunityLineItem(OpportunityId = opt.id, Product2Id = p1.Id, TotalPrice = 100, PricebookEntryId=customPrice.Id, Quantity =3);

Answer 2

First Insert the opportunity.
Then update the opportunity with the pricebookid.
// Insert Opportunity
Opportunity opt = new Opportunity(Name='Test',StageName='Prospect',
CloseDate=date.today(),BidType__c = 'Competitive',
Business_Line_BU__c = 'BL.619'
);
insert opt;
opt.PriceBook2 = customPB;
update opt;