So far we have learned how to write basic Drupal migrations and use process plugins to transform data meeting the format expected by the destination. In the previous chapter we learned one of many approaches to migrating images. Now we will change it a bit to introduce two new migration concepts: **constants** and **pseudofields**. Both can be used as data placeholders in the migration timeline. Along with other process plugins, they allow you to build dynamic values that can be used as part of the **migrate process pipeline**.
In the Migrate API, **source constants** are _arbitrary values that can be used later in the process pipeline_. They are set as direct children of the source section. You write a `constants` key whose value is a list of name-value pairs. Even though they are defined in the _source_ section, they are independent of the source plugin in use. The following code snippet shows a generalization for settings and using _constants_:
You can set as many constants as you need. Although not required by the API, writing the constants' names in all uppercase and using _underscores_ (**\_**) to separate words makes it easy to identify them. The value can be set to anything you need to use later. In the example above, there are strings, integers, decimals, and arrays. To use a constant in the process section you type its name, just like any other field provided by the _source_ plugin. Note that to use the constant you need to name the full hierarchy under the source section. That is, the word `constants` plus the name itself separated by a _slash_ (**/**) symbol. Their value can be used varbatim or transform via process plugins.
_Technical note_: The word `constants` for storing the values in the source section is not special. You can use any word you want as long as it does not collide with another configuration key of the source plugin in use. A reason to use a different name is if your source actually contains a field named `constants`. In that case you could use `defaults` or something else. The one restriction is that whatever value you use, you have to use it in the process section to refer to any constant. For example:
Similar to source constants, **pseudofields** store _arbitrary values for use later in the process pipeline_. There are some key differences. Pseudofields are set in the _process_ section. The name can be arbitrary as long as it does not conflict with a property name or field name in the destination. The value can be set to a verbatim copy from the _source_ (a field or a constant) or they can use process plugins for data transformations. The following code snippet shows a generalization for setting and using _pseudofields_:
In the above example, `_pseudo_field_1` is set to the result of a `concat` process transformation that joins a constant and a field from the source section. The result value is later used as part of a `urlencode` process transformation. Note that to use the value from `_pseudo_field_1` you have to enclose it in _quotes_ (**'**) and prepend an _at sign_ (**@**) to the name. The `_pseudo_` prefix in the name is not required. It is used to make it easier to distinguish among pseudofields and regular property or field names. The new value obtained from URL encode operation is stored in `_pseudo_field_2`. This last pseudofield is used to set the value of the `uri` subfield for `field_link`. The example could be simplified by using a single pseudofield and chaining multiple process plugins. It is presented that way to demonstrate that a pseudofield could be used as direct assignments or as part of process plugin configuration values.
_Technical note_: If the name of the subfield can be arbitrary, how can you prevent name clashes with destination property names and field names? You can look for an !!!online reference or review the class defining the entity and fields attached to it. In the case of a node migration, look at the `baseFieldDefinitions` method of the `Node` class for a list of property names. Be mindful of class inheritance and method overriding. For a list of fields and their machine names, look at the `Manage fields` section of the content type you are migrating into. The [Field API](https://api.drupal.org/api/drupal/core!modules!field!field.module/group/field/8.8.x) prefixes any field created via the administration interface with the string `field_`. This reduces the likelihood of name clashes. Other than these two name restrictions, _anything else can be used_. In this case, the Migrate API will eventually perform an entity save operation which will discard the pseudofields.
The migrate process pipeline is a mechanism by which the value of any **destination property**, **field**, or **pseudofield** that has been set **can be used by anything defined later in the process section**. The fact that using a pseudofield requires enclosing its name in _quotes_ and prepending an _at sign_ is actually a requirement of the process pipeline. Let’s see some examples using a node migration:
In the process pipeline, these values can be used just like constants and fields from the source. The only restriction is that they need to be set before being used. For those familiar with the _rewrite results_ feature of Views, it follows the same idea. You have access to everything defined previously. Anytime you use enclose a name in _quotes_ and prepend it with an _at sign_, you are telling the migrate API to look for that element in the process section instead of the source section.
Let’s practice the concepts of constants, pseudofields, and the migrate process pipeline by modifying the example of the previous chapter. The Migrate Files(!!!) module provides another process plugin named `image_import`. It allows you to directly set all the subfield values in the plugin configuration itself.
The code snippets will be compact to focus on particular elements of the migration. The full code is available at <https://www.drupal.org/project/migrate_examples> The module name is `Constants and Pseudofields Example` and its machine name is `constants_pseudofields_example`. This example uses the [Migrate Files](https://www.drupal.org/project/migrate_file) module. Make sure to download and enable it.
Only one record is presented to keep snippet short, but more exist. In addition to having a unique identifier, each record includes a name and details about the image. Note that this time, the `photo_url` does not provide an absolute URL. Instead, it is a relative path from the domain hosting the images. In this example, the domain is `https://udrupal.com` so that value is stored in the BASE_URL constant. This is later used to assemble a valid absolute URL to the image. Also, there is no photo description, but one can be created by concatenating some strings. The PHOTO_DESCRIPTION_PREFIX constant will be used to assemble a description.
The `title` node property is set directly to the value of the `name` field from the source. `_pseudo_image_url` stores a valid absolute URL to the image using the BASE_URL constant and the `photo_url`_field_ from the _source_. `_pseudo_image_description` uses the PHOTO_DESCRIPTION_PREFIX constant and the `name`_field_ from the _source_ to store a description for the image.
For the `field_image` field, the `image_import` process plugin is used. This time, the subfields are not set manually like in the previous chapter. The absence of the `id_only` configuration key allows you to assign values to subfields via the `image_import` plugin directly. The URL to the image is set in the `source` key and uses the `_pseudo_image_url` pseudofield. The `alt` key allows you to set the alternative attribute for the image using the `_pseudo_image_description` pseudofield. The `title` key expects the text to use for image's title. We are reusing the `title` node property which was set at the beginning of the process pipeline. Remember that destination properties, fields, and pseudofields are available as long as they were previosly defined in the pipeline. Finally, the `width` and `height` keys use fields from the source.
**Important**: By default, the migrate API will only expand the value of the `source` configuration. That is, replace its value either by a source field, source constant, or pseudofield. Any other configuration normally is not expanded and the its specified valued is passed verbatim to the process plugin. In the case of `image_import`, the plugin itself provides a mechanism to expand the values for the `alt`, `title`, `width`, and `height` configuration options. Most plugins do not this and will use the configured value literally.
