Prevent inserting document without rows

["Andrus" <eetasoft@xxxxxxxxx>]

I have tables for invoice headers and rows:

CREATE TABLE invoiceheader ( id SERIAL PRIMARY KEY, .... );

CREATE TABLE invoicerow ( id SERIAL PRIMARY KEY,
idheader INTEGER NOT NULL REFERENCES invoiceheader ON DELETE CASCADE ON 
UPDATE CASCADE,
 ....
 );

Each invoice must have at leat one row.

I want that transaction commit fails if invoice header is inserted without 
any row in invoice rows table.

How to prevent inserting invoice headers without rows ?

Andrus. 



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