When you migrate relations between two different entities, you usually migrate the target entities first and then the source entities.
But what if the target and source are of the same entity and bundle type? How do you ensure that the target entity gets migrated before the source entity?
Have you run into a chicken-versus-egg problem?
In this article, we’ll look at how to migrate hierarchical taxonomy terms to Drupal 8 or Drupal 9 by following along with an example migration. We’ll address the above questions to give you a good understanding of how to implement similar migrations in your projects.
The Drupal 8/9 Migration Tutorial Series
- Part 1: Migrating Basic Data
- Part 2: Migrating Taxonomy Terms and Term References
- Part 3: Migrating Files and Images
- New: Migrating Hierarchical Taxonomy Terms (you are here!)
- Coming soon: Migrating Media Items and Their Relationships
Before We Start
- If you are new to migrations in Drupal 8 or 9, you may want to read about migrating basic data to Drupal 8 first.
- You can follow the sample code for this tutorial on GitHub: Migrate example: Hierarchical Terms module on GitHub.
- You need to have migrate_tools, migrate_plus and migrate_source_csv modules to follow this tutorial.
- You need to have Drush installed to run migrations as described in this article.
- This tutorial is also valid for Drupal 9 migrations.
👩💻 Get up to speed on the latest version of Drupal! Ask us about the Upgrading to Drupal 9 & Beyond training.
The Problem
We have received these CSV files from our imaginary client:
- articles.csv: Contains the article data
- categories.csv: Contains category data and category relationships
We need to:
- Migrate categories including their relationships (parent categories) from categories.csv
- Migrate articles from articles.csv
Let’s get started.
Setting up the Migrations
Create the migration module
We need to create a module for our migrations. In this example, we’re naming it migrate_example_hierarchical_terms.
We then need to add the following modules as dependencies in the module declaration:
Create a migration group
To group the migrations, we also need to create a migration group. To do so, we’ll create a fairly simple configuration file so that the group gets created when the module is installed.
The file’s contents should be as follows:
id: hierarchical_terms label: Hierarchical Terms Group source_type: CSV
Writing the Migrations
Next thing to do is to write the actual migrations. For our requirements, we need to write two different migrations: one for categories and one for articles. The hierarchy stuff will be handled as part of the categories migration.
Since Drupal 8.1.x, migrations are plugins that should be stored in a migrations folder inside any module. You can still make them configuration entities as part of the migrate_plus module but I personally prefer to follow the core recommendation because it’s easier to develop (you can make an edit and just rebuild cache to update it).
Write the category migration
For the categories migration, we’re creating a categories.yml file inside the migrations folder. This migration uses CSV as source, so we need to declare it like this:
source: plugin: 'csv' path: 'modules/custom/migrate_example_hierarchical_terms/data/categories.csv' delimiter: ',' enclosure: '"' header_offset: 0 ids: - name fields: 0: name: name label: 'Name' ...
The destination part of the migration is pretty standard so we’ll skip it, but you can still look at it in the code.
For the process, the important part is the parent field:
parent: - plugin: migration_lookup migration: categories source: parent - plugin: default_value default_value: 0
We’re using the migration_lookup plugin and the current migration (categories) to look for the parent entities. We’re allowing stubs to be created (by not setting no_stub: true) so that if a child term is migrated before its parent term, the parent will be created as a stub and it will be completed later with the real data.
We’re also defaulting to 0 as parent if no parent is set in the source data. This way, the hierarchy will be preserved when running the migration.
Write the article migration
To migrate the articles, we’ve created the articles.yml migration file. If you have previous experience with migrations in Drupal 8, it’s pretty straightforward. It’s also using CSV as a source, so its source section is pretty similar to the one in the categories migration. The destination is set to be the article bundle of the node entity type.
The process section looks like this:
process: title: title body/value: content field_category: - plugin: migration_lookup migration: categories source: category
Title from the CSV file is mapped directly to title in the node. The content column in the CSV file is mapped to the value sub-field of the body field. For field_category, we’re also using the migration_lookup plugin to get the categories that we’ve previously migrated.
We’re also setting a dependency to the categories migration to ensure that categories run before articles:
migration_dependencies: required: - categories
Now, everything is in place and we’re ready to run the migrations.
Running the Migrations
Given that we have set dependencies, we can instruct Drupal to run the migration group and it will run the migrations in the right order.
To do so, execute drush mim --group=hierarchical_terms. The output will look like this:
[notice] Processed 11 items (7 created, 4 updated, 0 failed, 0 ignored) - done with 'categories' [notice] Processed 10 items (10 created, 0 updated, 0 failed, 0 ignored) - done with 'articles'
Note that the counts for categories are not what you’d expect looking at the data. This is because of the stub creation that happened during the migration. However, if you run drush ms, the output will be as expected:
----------------------------------------------- -------------- -------- ------- ---------- ------------- --------------------- Group Migration ID Status Total Imported Unprocessed Last Imported ----------------------------------------------- -------------- -------- ------- ---------- ------------- --------------------- Hierarchical Terms Group (hierarchical_terms) categories Idle 9 9 0 2020-08-21 19:18:46 Hierarchical Terms Group (hierarchical_terms) articles Idle 10 10 0 2020-08-21 19:18:46 ----------------------------------------------- -------------- -------- ------- ---------- ------------- ---------------------
Next Steps
- Check out the code for the migrate_example_hierarchical_terms module on GitHub.
- Read about migrating basic data to Drupal 8.
- Learn more about migrations on our blog.
- Sign up for one of our advanced Drupal training courses to step up your Drupal development skills.
