arel, how to join

Question

Given

class Category < ActiveRecord::Base
  has_many :products, :order => 'name ASC'
end

Using the Rails 3 stack, how can I query for all categories that 'have' products?

Solution 1

Category.joins(:products).select("distinct categories.*").all

Solution 2

In ARel (NOT ActiveRecord) we will do the following:

p = Arel::Table.new :products    # Base Rel-var
c = Arel::Table.new :categories  # Base Rel-var

predicate = p[:category_id].eq( c[:id] ) # for equality predicate

p.join(c)                   # Natural join
  .on( predicate )          # Equi-Join
  .group( p[:category_id] ) # Grouping expression to get distinct categories
  .project( c[:id] )        # Project the distinct category IDs of the derived set.

Solution 3

Another, simpler, approach is to use the ActiveRecord query interface's join in conjunction with ARel for the conditional statement:

joins(:user)
.where(User.arel_table[:name].matches("%#{query}%"))

Generates the following sql in sqlite3:

"SELECT \"patients\".* FROM \"patients\" INNER JOIN \"users\" ON \"users\".\"id\" = \"patients\".\"user_id\" WHERE (\"users\".\"name\" LIKE '%query%')"

And the following sql in postgres (notice the ILIKE):

"SELECT \"patients\".* FROM \"patients\" INNER JOIN \"users\" ON \"users\".\"id\" = \"patients\".\"user_id\" WHERE (\"users\".\"name\" ILIKE '%query%')"

This allows you to join with simplicity, but still get the abstraction of the ARel matcher to your RDBMS.

Comments

[Jan]

Do you happen to know a good reference on what is possible with arel queries?

[gertas]

I take my knowledge from googling, reading Rails-elite blogs. Important thing: ActiveRecord differs much from pure Arel.

[Translunar]

When you say "In ARel (NOT ActiveRecord)," what precisely do you mean? That we can't do this in a class derived from ActiveRecord::Base? This is confusing.

[bradgonesurfing]

In all these examples I never see the final step folding the AREL query back into Rails to get real records. What to do with p now? You can call to_sql on it for sure but how to turn it into an ActiveRecord::Relation that will load records?

[Tim Fletcher]

@bradgonesurfing Just stick it in an AR where method. ARel just generates SQL.

[bradgonesurfing]

The above is a join and can't be stuck in a where method.

[sockmonk]

For an arel that generates a SELECT (not just a where clause), you use find_by_sql(arel_expression.to_sql). This will return an array of records, not a chainable ActiveRecord::Relation, but at least it gives you the results.

[Harry Wood]

@bradgonesurfing is asking a different question here really. Here is that question, and its answer: stackoverflow.com/questions/13442073/… Briefly, a good answer is to use .all.each, and loop (Using find_by_sql works, but is not a good answer) That's a basic ActiveRecord thing, which is why people tend to leave out these next steps, when answering questions like this about ARel querying.