title: Firebird autoIncrement issue
body: <p>I've created Customers Table through following code :</p>
  
  <pre><code>CREATE TABLE CUSTOMERS (
  ID INTEGER DEFAULT 1 NOT NULL,
  "NAME" VARCHAR(30) CHARACTER SET UTF8 COLLATE UTF8,
  "LASTNAME" VARCHAR(30) CHARACTER SET UTF8 COLLATE UTF8);
  
  
  ALTER TABLE CUSTOMERS ADD PRIMARY KEY (ID);
  
  
  SET TERM ^ ;
  
  CREATE TRIGGER BI_CUSTOMERS_ID FOR CUSTOMERS
  ACTIVE BEFORE INSERT
  POSITION 1
  AS
  BEGIN
   IF (NEW.ID IS NULL) THEN
    NEW.ID = GEN_ID(CUSTOMERS_ID_GEN, 1);
  END^
  
  SET TERM ; ^
  </code></pre>
  
  <p>But when I inserting second row like :</p>
  
  <pre><code>insert into Customers(Name,LastName) values('Hamed','Kamrava');
  </code></pre>
  
  <p>It gets below error :</p>
  
  <pre><code>Violation of PRIMARY or UNIQUE KEY constraint "INTEG_2" on table "CUSTOMERS".
  </code></pre>