So far, our Field Type’s value is represented by the Tweet\Value class. It holds a semantic representation of the type’s data: a url, author url and the tweet's content.

The next step is to tell the system how to actually store this.

About converters

Unlike eZ Publish Legacy, eZ Platform supports (by design) multiple storage engines. The main, and almost only one right now is the Legacy Storage Engine, based on the legacy database, with a new implementation. Since each storage engine may have its own way of storing data, we need to map each Field Type value to something legacy can understand. To do this, we will implement a Field Type Converter, each storage engine defining its own interface for those.

Legacy Field Type converters

The legacy storage engine uses the ezcontentobject_attribute table to store Field values, and ezcontentclass_attribute to store Field definition values (settings, etc.). They're both based on the same principle. Each row represents a Field or a FieldDefinition, and offers several free fields, of different types, where the type can store its data:

• ezcontentobject_attribute offers three fields for this purpose: data_int , data_text and data_float
• ezcontentclass_attribute offers a few more: four data_int (data_int1 to data_int4) fields, four data_float  (data_float1 to data_float5) ones, and five data_text (data_text1 to data_text5) .

Each type is free to use those fields in any way it requires. Converters will map a field’s semantic values to the fields described above, for both settings (validation + configuration) as well as value.

Implementing Tweet\LegacyConverter

The Converter will be placed along with the Type and Value definitions (the Kernel stores them inside the Legacy Storage Engine structure): eZ/Publish/FieldType/Tweet/LegacyConverter.php . A Legacy Converter must implement the eZ\Publish\Core\Persistence\Legacy\Content\FieldValue\Converter interface:

The Converter interface expects us to implement five methods:

• toStorageValue() and toFieldValue()
  used to convert an API field value to a legacy storage value, and a legacy storage value to an API field value.  

• toStorageFieldDefinition() and toFieldDefinition()
  used to convert a field definition to a legacy one, and a stored legacy field definition to an API field definition.

• getIndexColumn()
  Tell the API which legacy DB field should be used to sort & filter content, either sort_key_string or sort_key_int

Implementing Field Value converters: toFieldValue() and toStorageValue()

As said above, those two methods are used to convert from a Field to a value that Legacy can store, and the other way around.

We have defined that we wanted to store the tweet’s URL in data_text, and that sorting would be done on the username-status-tweetid string we extract in getName() and getSortInfo(). 

toStorageValue() will fill the provided eZ\Publish\Core\Persistence\Legacy\Content\StorageFieldValue from a Tweet\Value, while toFieldValue() will do the exact opposite:

With these two methods, the legacy storage engine is able to convert a Tweet\Value into legacy data, and legacy data back into a Tweet\Value object.

Implementing Field Definition converters: toStorageFieldDefinition() and toFieldDefinition()

The first two methods we have implemented apply to a Field’s value . But we also need to convert our Field’s definition. For example, a TextLine’s max length, or any FieldDefinition option.

This is done using toStorageDefinition() that converts a FieldDefinition into a StorageFieldDefinition. toFieldDefinition() does the opposite. In our case, we actually don’t need to implement those methods since our Tweet Type doesn’t have settings:

Implementing getIndexColumn()

In toFieldValue() and toStorageValue() we have used the sortKeyString property from StorageFieldValue. getIndexColumn() will tell provide the legacy storage engine the type of index / sort column it should use: string (sort_key_string) or int (sort_key_int). Depending on which one is returned, the system will either use the sortKeyString or the sortKeyInt properties from the StorageFieldValue.

Registering the converter

Just like a Type, a Legacy Converter needs to be registered and tagged in the service container.

The tag is ezpublish.storageEngine.legacy.converter, and it requires an alias attribute to be set to the Field Type identifier (eztweet). Let's add this block to  Resources/config/services.yml: