Everything I read in your posts suggest that you’re finding the role of Changesets confusing.

Changesets are all about validating that a set of proposed changes meet the criteria that you set. There’s a little bit more to it than that in that often times a Changeset will determine if there’s even a change at all being made as compared to a starting data struct you provide, but not much more than that. A Changeset is not really involved in questions of data retrieval. Preloads on the other hand are involved in data retrieval.

So, for example, in the Changeset you posted, the validate_required/3 call you made says that any proposed change that is validated by the Changeset must include values for :value and :description. These values may come from the starting data or be part of the change you’re validating, but they must have a value. The cast/4 and cast_assoc/3 calls are doing the initial comparison of the posts argument (the original or starting data struct, the struct before the proposed changes) and the attrs argument which is the map containing the changes you want to make to posts. The casts produce an initial Changeset struct which has figured out what exactly is changing (if anything). Eventually the Changeset struct is passed on a function like insert/2 or update/2 and if all of the validations in the Changeset were passed, the insert or update takes place… if the Changeset failed in one of its requirements, then the insert/update doesn’t happen and you get a corresponding failure return result from those functions.

preload/3 on the other hand is about data retrieval. As others have mentioned, associated data is not automatically loaded in a query, you have to ask for it to be “preloaded”. Repo.preload/3 is used after you have already retrieved data from a query but without preloaded associations at query time: you can ask for that data after the fact. Ecto.Query.preload/3 allows you to ask for the preload in the query itself (see Ecto.Query.from/2). Changesets are not used in data retrieval so preload doesn’t apply.

Our answers may have been a bit confusing, too, because while Changesets aren’t involved in data retrieval, an insert or update can return a data struct if requested (and if using a database that supports it) via the returning: true option. If you get data back from the insert or update that way, I would expect that you could then use Repo.preload/3 to load the associations from that returned data.

As a bit of additional context: Differently to other ORMs or db libraries in various languages ecto does not automatically load relationships. Those fields by default are populated with the Ecto.Association.NotLoaded struct and are only filled with real data once you explicitly load the associations using preload – giving you the tools to prevent n+1 queries from the start.

This error is likely coming from a template somewhere. When your ProberParameterfileAssembler.Parameter.Posts struct does not have the parameterset association loaded the value of the :parameterset key on that struct would be an Ecto.Association.NotLoaded struct.

If you look in your templates for something like <%= @post.parameterset %> you can probably find the location where the error is raised.

In order to display the parameterset you will need to load the association, possibly by using Repo.preload/3 which is documented here and then render the values.

<%= @post.paremeterset.address %>
<%- @post.parameterset.description %>