Improve error messages for invalid assocs, closes #3333

lib/ecto/changeset.ex

@@ -862,8 +862,10 @@ defmodule Ecto.Changeset do
 
   defp relation!(_op, type, _name, {type, relation}),
     do: relation
+  defp relation!(op, :assoc, name, nil),
+    do: raise(ArgumentError, "cannot #{op} assoc `#{name}`, assoc `#{name}` not found. Make sure it is spelled correctly and that the association type is not read-only")
   defp relation!(op, type, name, nil),
-    do: raise(ArgumentError, "cannot #{op} #{type} `#{name}`, assoc `#{name}` not found. Make sure it is spelled correctly and properly pluralized (or singularized)")
+    do: raise(ArgumentError, "cannot #{op} #{type} `#{name}`, #{type} `#{name}` not found. Make sure that it exists and is spelled correctly")
   defp relation!(op, type, name, {other, _}) when other in @relations,
     do: raise(ArgumentError, "expected `#{name}` to be an #{type} in `#{op}_#{type}`, got: `#{other}`")
   defp relation!(op, type, name, schema_type),

lib/ecto/schema.ex

@@ -847,17 +847,6 @@ defmodule Ecto.Schema do
       post = Repo.get(Post, 42)
       authors = Repo.all assoc(post, :comments_authors)
 
-  Although we used the `:through` association in the example above, Ecto
-  also allows developers to dynamically build the through associations using
-  the `Ecto.assoc/2` function:
-
-      assoc(post, [:comments, :author])
-
-  In fact, given `:through` associations are read-only, **using the `Ecto.assoc/2`
-  format is the preferred mechanism for working with through associations**. Use
-  the schema-based one only if you need to store the through data alongside of
-  the parent struct, in specific cases such as preloading.
-
   `:through` associations can also be preloaded. In such cases, not only
   the `:through` association is preloaded but all intermediate steps are
   preloaded too:
@@ -882,6 +871,8 @@ defmodule Ecto.Schema do
       [comment] = Repo.all(Comment) |> Repo.preload(:post_permalink)
       comment.post_permalink #=> %Permalink{...}
 
+  Note `:through` associations are read-only. For example, you cannot use
+  `Ecto.Changeset.cast_assoc/3` to modify through associations.
   """
   defmacro has_many(name, queryable, opts \\ []) do
     queryable = expand_alias(queryable, __CALLER__)