Ecto: How to preload records with selecting another joined columns

Question

Are there any ways to preload records by selecting another joined columns?

# table structure
# User 1---* Post 1---* PostTag *---1 Tag

# extract definition of s         


        

Answer 1

As the error message says, you need to select the binding you gave in from when you are preloading, otherwise Ecto has no place to put the preloaded tags. Here is a simple answer:

query = from post in Post,
  join: user in User, on: post.user_id == user.id,
  select: {post, user.name},
  preload: [:tags]

By returning a tuple, you can have the full post and the user.name on the side. Another approach is to return both post and users as full structs:

query = from post in Post,
  join: user in User, on: post.user_id == user.id,
  preload: [:tags, user: user]

or if you don't want all fields:

query = from post in Post,
  join: user in User, on: post.user_id == user.id,
  preload: [:tags, user: user],
  select: [:id, :title, :user_id, user: [:name]]