Take maxims such as "thy model shall only care about the database stuff" with a grain of salt. Dogma has no place in engineering.
In order to answer this question you need to first understand the nature of this validation array, and whether it fits conceptually with the model or with controlling the model.
The fact that these names are stored inside an array is inconsequential; you could be sending names out to a serial port and receiving back "yes" or "no" answers for all you care. This could still legitimately be part of your model. The question is: does the task of validation sound to you more like "controlling" the model, or like querying the model?
You have not provided enough details about this mysterious array, but I am tempted to believe that it is 100% part of the model. The names that it contains could have easily been stored in the database, but for some reason someone decided not to. If you narrowly think of "the model" as "the database", you might be mislead to think that they don't belong to the model, but if you realize that your model is your representation of all of your entities and the relationships between them, then perhaps it might begin to make sense that the contents of that array are simply describing a relationship which for some reason was not encoded in the database.
After the amendment to the question, I would say with even greater confidence that the list of items (and therefore the list of their ids) is part of the model, though I still cannot say it with complete confidence, since it is still not clear to me what is the nature of these entities and what is the domain that the database describes.
It seems to me, however, that what you are missing is an abstraction layer which combines the database and the API and whatever else you might have into a uniform model which looks homogeneous and spares the rest of your code from having to be concerned about different sources for different subsets of the information that it works with. Conceptually, if these entities have ids, and these ids are stored in the database, then they are all part of the same model, regardless of where they come from.