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Working With Existing Databases

Working With Existing Databases

The following topics are targeted for developers who already have a working database solution in place, but would like to use Propel to work with the data. For this case, Propel provides a number of command-line utilities helping with migrations of data and data structures.

Working with Database Structures

Propel uses an abstract XML schema file to represent databases (the schema). Propel builds the SQL specific to a database based on this schema. Propel also provides a way to reverse-engineer the generic schema file based on database metadata.

Creating an XML Schema from a DB Structure

To generate a schema file, create a new directory for your project & specify the connection information in your build.properties file for that project. For example, to create a new project, legacyapp, follow these steps:

  1. Create the legacyapp project directory anywhere on your filesystem:

{% highlight bash %}

mkdir legacyapp cd legacyapp {% endhighlight %}

  1. Create a build.properties file in legacyapp/ directory with the DB connection parameters for your existing database, e.g.:

{% highlight ini %} propel.project = legacyapp

The Propel driver to use for generating SQL, etc.

propel.database = mysql

This must be a PDO DSN

propel.database.url = mysql:dbname=legacyapp propel.database.user = root

propel.database.password

{% endhighlight %}

  1. Run the reverse task to generate the schema.xml:

{% highlight bash %}

propel-gen reverse {% endhighlight %}

  1. Pay attention to any errors/warnings issued by Phing during the task execution and then examine the generated schema.xml file to make any corrections needed.

  2. _'You're done! _' Now you have a schema.xml file in the legacyapp/ project directory. You can now run the default Propel build to generate all the classes.

The generated schema.xml file should be used as a guide, not a final answer. There are some datatypes that Propel may not be familiar with; also some datatypes are simply not supported by Propel (e.g. arrays in PostgreSQL). Unfamiliar datatypes will be reported as warnings and substituted with a default VARCHAR datatype.

Tip
The reverse engineering classes may not be able to provide the same level of detail for all databases. In particular, metadata information for SQLite is often very basic since SQLite is a typeless database.

Migrating Structure to a New RDBMS

Because Propel has both the ability to create XML schema files based on existing database structures and to create RDBMS-specific DDL SQL from the XML schema file, you can use Propel to convert one database into another.

To do this you would simply:

  1. Follow the steps above to create the schema.xml file from existing db.
  2. Then you would change the target database type and specify connection URL for new database in the project's build.properties file:

{% highlight ini %} propel.database = pgsql propel.database.url = pgsql://unix+localhost/newlegacyapp {% endhighlight %}

  1. And then run the sql task to generate the new DDL:

{% highlight bash %}

propel-gen sql {% endhighlight %}

  1. And (optionally) the insert-sql task to create the new database:

{% highlight bash %}

propel-gen insert-sql {% endhighlight %}

Working with Database Data

Propel also provides several tasks to facilitate data import/export. The most important of these are datadump and datasql. The first dumps data to XML and the second converts the XML data dump to a ready-to-insert SQL file.

Tip
Both of these tasks require that you already have generated the schema.xml for your database.

Dumping Data to XML

Once you have created (or reverse-engineered) your schema.xml file, you can run the datadump task to dump data from the database into a data.xml file.

{% highlight bash %}

propel-gen datadump {% endhighlight %}

The task transfers database records to XML using a simple format, where each row is an element, and each column is an attribute. So for instance, the XML representation of a row in a publisher table:

publisher_id name
1 William Morrow

... is rendered in the data.xml as follows:

{% highlight xml %} ... ... {% endhighlight %}

Creating SQL from XML

To create the SQL files from the XML, run the datasql task:

{% highlight bash %}

propel-gen datasql {% endhighlight %}

The generated SQL is placed in the build/sql/ directory and will be inserted when you run the insert-sql task.

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