Gotchas in the Ruby Sequel Gem

| 2 min read

I haven’t really used Sequel much therefore I am definitely a newbie. However, after days and nights of frustration, endless debugging and some search-fu during the development of Datamappify, I have finally arrived at the conclusion that Sequel is a capable library, as long as you are aware of the gotchas.

Gotcha 1: Always use “select“/“select_all“, or your data records will mysteriously have wrong IDs!

In ActiveRecord, joining an associated model couldn’t be simpler:


In Sequel, despite having a similar API for models to declare associations and their corresponding primary and foreign keys, you cannot do a join without specifying the keys:

Not good:

# or


Post.join(:authors, :id => :author_id)

You would think the version above works - it doesn’t. Even worse, the above example will give you incorrect data - the IDs of the Post records will now contain the IDs from their corresponding Author records! This is because upon a join, Sequel merges attributes from both models into a single hash.

The correct version:

Post.join(:authors, :id => :author_id).select(:posts __id, :posts__ title, :posts__body)
# or
Post.join(:authors, :id => :author_id).select_all(:posts)

Gotcha 2: Always call “all“ at the end of the chain, or the chain will present data in a different format.

In ActiveRecord, all of the below examples return an ActiveRecord::Relation collection:

Post.where(:title => 'Hello world')

And indeed, calling first on any of them returns an object of class Post (assuming the result collection is not empty).

Post.where(:title => 'Hello world').first.class #=> Post
Post.joins(:author).first.class #=> Post
Post.includes(:author).first.class #=> Post

In Sequel, the below examples all return a Sequel::DataSet collection:

Post.where(:title => 'Hello world')

But let’s see what we get from calling first.class on them:

Post.where(:title => 'Hello world').first.class #=> Post
Post.eager(:author).first.class #=> Post
Post.eager_graph(:author).first.class #=> Hash

Huh? Last one is a Hash? It turns out, if you call all at the end of chains to convert them to Arrays, then the returned collections are consistent:

Post.where(:title => 'Hello world').all.first.class #=> Post
Post.eager(:author).all.first.class #=> Post
Post.eager_graph(:author).all.first.class #=> Post

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