Can you print an example of the random test error? I run a decent sized phoenix project (150K lines) and likely can help you out if you give a bit more detail.

Can you print an example of the random test error? I run a decent sized phoenix project (150K lines) and likely can help you out if you give a bit more detail. With regard to associations, for simple belongs_to associations I will usually write separate changeset functions where the first argument is pattern matched against the record. I will give an example with Post/Comment defmodule MyApp.Blog.Comment do def changeset(%Post{} = record, params) do record |> build_assoc(:comments) |> changeset(params) end def changeset(%Comment{} = record, params) do record |> cast(params, [:body]) |> validate_required([:body]) # Continue with validations, constraints, etc. end end In this case the changeset function calls itself once it handles building the association. For more complex conditions, I will usually write a "Form" module that may handle that case usually with a Enum.reduce combining multiple calls to `put_assoc` so that I can make nested associations in one insert. Hope this helps a bit.