How to add new column with default value from exis

问题:

I'm using Postgres DB and for migration I'm using Liquibase. I have an ORDERS table with the following columns:

ID | DATE | NAME | CREATOR | ...

I need to add a new column which will hold the user who has last modified the order - this column should be not-nullable and should have default value which is the CREATOR. For new orders I can solve the default value part of the business logic, but thing is I already have an existing orders and I need to set the default value when I create the new column. Now, I know I can set a hard-coded default value in Liquibase - but is there a way I could add the default value based on some other column of that table (for each entity).

回答1:

Since no one answered here I'm posting the way I handled it:

<changeSet id="Add MODIFY_USER_ID to ORDERS" author="Noam">
        <addColumn tableName="ORDERS">
            <column name="MODIFY_USER_ID" type="BIGINT">
                <constraints foreignKeyName="ORDERS_MODIFY_FK" referencedTableName="USERS" referencedColumnNames="ID"/>
            </column>
        </addColumn>
</changeSet>

<changeSet id="update the new MODIFY_USER_ID column to get the CREATOR" author="Noam">
    <sql>update ORDERS set MODIFY_USER_ID = CREATOR</sql>
</changeSet>

<changeSet id="Add not nullable constraint on MODIFY_USER_ID column" author="Noam">
    <addNotNullConstraint tableName="ORDERS" columnName="MODIFY_USER_ID" columnDataType="BIGINT"/>
</changeSet>

I've done this in three different change-sets as the documentation recommends

回答2:

You could use the defaultValueComputed attribute, which takes the name of a procedure or function. You would have to also create a changeset that creates the procedure.

That might look something like this:

<changeSet author="steve" id="createProcedureForDefaultValue">
    <createProcedure procedureName="myCoolProc">
    CREATE OR REPLACE PROCEDURE myCoolProc IS
    BEGIN
       -- actual logic here
    END;
    </createProcedure>
</changeSet>

<changeSet author="steve" id="addDefaultValueColumn">
    <addColumn tableName="ORDERS">
        <column name="LAST_MODIFIED_BY" type="VARCHAR" defaultValueComputed="myCoolProc">
            <constraints nullable="false"/>
        </column>
    </addColumn>
</changeSet>

Alternatively, you could do this using the <sql> tag.