Advanced xPDO Relationship Criteria

Sometimes you need to define multiple criteria in xPDO schemas to make sure your relations are properly defined. I came across this while working on a project that needed some existing databases integrated. This specific case had one "fl_translations" table that contained the translations for thousands of objects. To get the translations for a specific object, you would need to filter that table on the "originaltable" field. For example, one row might have originaltable=fl_colors and contains the translation for the color in a specific language.

In order to properly define this relation, I went browsing the MODX core schema for some ideas, and I stumbled across a pretty much undocumented feature of xPDO Schemas: relation criteria!

Here's a snippet of the final schema:

Basically, while with a normal relation you would immediately close the <composite> (or <aggregate>) element, but if you want additional filters (criteria) on the relation, you can define that. A simple <criteria> object with a target (I presume but haven't tested that you can add an additional <criteria> object with a target of local), and inside it a JSON object with the fieldname (of the foreign object) and the value it should have (in this case, fl_colors, as that's how it was set up).

This way of defining relations is very powerful and I imagine some really complicated relations could be defined once (in the schema) and reused without knowing all the details by simply calling getMany('RelationAlias').

The core uses this type of relationship for defining the PropertySets relations on elements and also in the modAccess definition. For example, here is a snippet from the core schema for modChunk:

xPDO is quite nifty eh?


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addOne or addMany, when to use which

Just a quick post here for something that I see asked (or, more accurately, see being done in the wrong way) way too often.

When you're building your own components or data model using xPDO Schemas, you will probably be working with that data in processors, snippets or external applications as well. At some point you will have related tables, and you will love coming across methods like addOne and addMany - they're awesome!

There's one big caveat when using them, and that is to figure out which one to use.

Oh, you use addOne when you want to add *one* related object, and addMany when you want to add multiple in one go? WRONG. Well, okay - there's a small truth in there. But it is not related to the fact if you're in the process of adding only one related object or an array of objects. If you are using the wrong method you can often see this by the lack of relation being added to the extra object(s), like the relation is not "sticking" for some reason.

So do I use addOne or addMany?

If you've read the Defining Relationships documentation page, which is a great introduction to aggregate and composite relations with xPDO schemas, you should have noticed this thing called the cardinality. This little thing is very important and the key to figuring out when you need addOne, and when you need addMany.

You see, if the cardinality is one, you will need to use addOne. If the cardinality is many, you will need to use addMany. And that's all folks.

That's all folks!

Teaser image by Sepehr Ehsani.


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Exploring modObjectGetListProcessor, a powerful class based processor in MODX 2.2

As of MODX Revolution 2.2, developers are handed class-based processors to speed up development of the back-end components. These are great, and I have blogged about Class Based Processors in general before with some quick examples, but in this article we'll dive into a particularly awesome one: modObjectGetListProcessor.

The modObjectGetList processor is mostly used for populating grids through the modExt Grid implementation, but you could also use it for any other widget that uses a JSON Data Store. And processors aren't limited to being used by connectors for back-end components.. they're also great to keep your code DRY (Don't Repeat Yourself) for use in Snippets!

For this article we'll assume a simple grid though. The techniques displayed can be used to point you in the right direction for other implementations.

The Basics

Here's basically the minimum processor file you can use:

.. and the reason they're so awesome. Brief, super awesome, working code!

The most important thing to note is the public variable $classKey, this is the class name of the object you are going to retrieve. Furthermore you'll see we define the $defaultSortField to the "initiatedon" date field from the schema, and with $defaultSortDirection we make sure we get the latest on top. The $objectType variable is not necessarily required, but allows you to use prefix (lexicon) error messages for some default sanity checks. For example in the update processor it will use the objectType to prefix _err_ns if the primary key is not specified.

We also make sure we return the name of our extended class in the end, as that is used to instantiate the processor when it's called. While you're free to name it whatever you want, I'd advise you to keep it the same as your classKey. That way, when adding a new processor, you can just copy/paste another one and find/replace the old classKey for the new one and you're good to go.

Exploring the Processor Process

Just like an earlier post, dealing with the modObjectUpdateProcessor class based processor, I have created a list of what happens in the processor that you will find below.

  1. Processor instantiated, properties being set.
  2. Using checkPermissions() the processor decides if the user is allowed to access it.
  3. The processor finds lexicon topics to load via getLanguageTopics, which gets its data from the languageTopics variable (as an array).
  4. initialize() is called on the processor, which sets a number of default properties including sort to the defaultSortField (default: name) class variable, and the direction to the defaultSortDirection variable (default: ASC).
  5. process() is called.
  6. beforeQuery() is triggered by process(), and if the result is not a boolean TRUE it will consider the processor to have failed and cancel further execution.
  7. getData() is triggered by process().
  8. The getData() method builds an xPDOQuery object for the classKey type.
  9. The getData() method calls prepareQueryBeforeCount(xPDOQuery $c) allowing you to add additional conditions to the queries. After calling that, it fetches the total amount of results using modX.getCount.
  10. prepareQueryAfterCount(xPDOQuery $c) is called by getData().
  11. The query is sorted with help of the getSortClassKey() method, and the sortAlias, sort and dir properties.
  12. If the limit property is larger than 0 it limits the query and sets an offset.
  13. modX.getCollection is called with your data, it's been retrieved.
  14. Every row is iterated over using the iterate(array $data) method. iterate calls beforeIteration(array $list), and starts looping over the rows.
  15. If the checkListPermission variable is true, the object extends modAccessibleObject and checkPolicy('list') is false, it skips the row.
  16. prepareRow(xPDOObject|modAccessibleObject $object) is called which needs to return an array with the objects' fields. Great method to customize the retrieved data. The array is added to the list.
  17. After iteration over the entire result set afterIteration(array $list) is called.
  18. The data is returned.

Example Usages

Defining Constraints (where field X has value Y)

When adding constraints, we will take our minimum processor and add (actually override) a new function called prepareQueryBeforeCount. This function takes in the xPDOQuery object as parameter, and expects it to be returned as well.

Easy enough we first get the "reservation" value using $this->getProperty(). By specifying a second value we are assigned a default instead of NULL. In this case I'm setting the default value to zero, which makes sure that if there is no reservation passed, it will not return any results - but no results instead (as all rows have a reservation set to > 0).

After getting the reservation variable, we just interact with the xPDOQuery $c as we would in a normal processor (or script) and pass our where condition.

In the end we return the xPDOQuery (this is important!) and we've limited our query to just that reservation.

Modifying the way row data is returned

In some cases, your database set up may not completely match how you want to display that data in the front end. For example, you may have an array (which is stored serialized), which you want returned as one line of text per array key=>value, for rending in a textarea for example.

You will also see that instead of calling simply $object->toArray(), I am passing some additional parameters.

Specifically selecting fields

You could also join tables in the prepareQueryBeforeCount processor, add additional constraints etc.

Are there any more examples you would like to see, or have some to share yourself? Let me know in the comments!


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