When can I use a custom field type instead of a custom entity?

Question

I come from an SQL background and I'm still navigating fields/entities. I have a custom entity (analysis) to record information about a laboratory sample (what it is, when and where it was taken, and what analysis method was applied to it). The analysis will return many independent results. I was thinking of making a second entity (results) with an entity reference to the analysis entity, the name of the chemical that was measured, and the concentration that was measured.

However, could I instead make a custom field_type that is combination of the chemical name and its concentration? Then I could include this chemical_result as a field in my analysis entity with

->setCardinality(FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED)

to include multiple chemical results in one analysis, right? When is it appropriate to use a custom field versus an entity?

I also have an entity called "chemical" that has information on chemicals of interest. If I use an entity for my results, I know I can make an entity_reference to the chemical entity, which would allow some more complex queries down the road. Can I do the same with a custom field? If not directly, I'm assuming I could program the add_analysis form to only allow certain values for chemical name when adding data to the result field, correct?

Answer 1

Custom entities and custom fields are not mutually exclusive, you can use them together.

You can build a custom field type with more than one property. See for example the Password field which defines three properties:

https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Field%21Plugin%21Field%21FieldType%21PasswordItem.php/function/PasswordItem%3A%3ApropertyDefinitions/8.2.x

You can also extend the entity references field. See for example the File field:

https://api.drupal.org/api/drupal/core%21modules%21file%21src%21Plugin%21Field%21FieldType%21FileItem.php/function/FileItem%3A%3ApropertyDefinitions/8.2.x

The File field adds two properties to the reference field. In your case you would add the properties chemical name and concentration to a reference field which points at the analysis entity.