New Topic: Idea for checking preconditions during updateSQL

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A new topic, 'Idea for checking preconditions during updateSQL', has been made on a board you are watching.

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http://liquibase.org/forum/index.php?topic=177.new#new

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This was going to be another possible extension contest entry, but day-job demands got in the way...

Here's an idea for checking preconditions during updateSQL:

Create a DatabaseWrapper class which encapsulates a rolling DatabaseSnapshot and a regular Database.

Build an initial DatabaseSnapshot at startup.

Use new custom SqlGenerators to modify the rolling DatabaseSnapshot as appropriate for the statement (e.g. CreateTable, AddColumn, etc).

When a precondition asks for a DatabaseSnapshot, let the Wrapper serve up the rollingDatabaseSnapshot instead.

That way, if you have precondition dependencies, your updateSQL should take those into account.

For example,

Code:
   
   <changeSet id="1" author="cimershein">
         <createTable tableName="person">
            <column name="id" type="int" autoIncrement="true">
                <constraints primaryKey="true" nullable="false"/>
            </column>
            <column name="firstname" type="varchar(50)"/>
            <column name="lastname" type="varchar(50)">
                <constraints nullable="false"/>
            </column>
        </createTable>
    </changeSet>

    <changeSet id="2" author="cimershein">
        <preConditions>
            <tableExists tableName="person"/>
        </preConditions>
         <addColumn tableName="person">
            <column name="username" type="varchar(8)"/>
        </addColumn>
    </changeSet>


Chris Imershein

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