Combine two ActiveRecord::Relation objects

Relation objects can be converted to arrays. This negates being able to use any ActiveRecord methods on them afterwards, but I didn't need to. I did this:

name_relation = first_name_relation + last_name_relation

Ruby 1.9, rails 3.2

merge actually doesn't work like OR. It's simply intersection (AND)

I struggled with this problem to combine to ActiveRecord::Relation objects into one and I didn't found any working solution for me.

Instead of searching for right method creating an union from these two sets, I focused on algebra of sets. You can do it in different way using De Morgan's law

ActiveRecord provides merge method (AND) and also you can use not method or none_of (NOT).

search.where.none_of(search.where.not(id: ids_to_exclude).merge(search.where.not("title ILIKE ?", "%#{query}%")))

You have here (A u B)' = A' ^ B'

UPDATE: The solution above is good for more complex cases. In your case smth like that will be enough:

User.where('first_name LIKE ? OR last_name LIKE ?', 'Tobias', 'Fünke')

I've been able to accomplish this, even in many odd situations, by using Rails' built-in Arel.

User.where(
  User.arel_table[:first_name].eq('Tobias').or(
    User.arel_table[:last_name].eq('Fünke')
  )
)

This merges both ActiveRecord relations by using Arel's #or.

Merge, as was suggested here, didn't work for me. It dropped the 2nd set of relation objects from the results.

There is a gem called active_record_union that might be what you are looking for.

It's example usages is the following:

current_user.posts.union(Post.published)
current_user.posts.union(Post.published).where(id: [6, 7])
current_user.posts.union("published_at < ?", Time.now)
user_1.posts.union(user_2.posts).union(Post.published)
user_1.posts.union_all(user_2.posts)

This is how I've "handled" it if you use pluck to get an identifier for each of the records, join the arrays together and then finally do a query for those joined ids:

  transaction_ids = @club.type_a_trans.pluck(:id) + @club.type_b_transactions.pluck(:id) + @club.type_c_transactions.pluck(:id)
  @transactions = Transaction.where(id: transaction_ids).limit(100)

If you have an array of activerecord relations and want to merge them all, you can do

array.inject(:merge) # like where AND

array.inject(:or) # like where OR

In the case where using or is not compactible, I use something like this to get the ActiveRecord_Relation object

User.from("(#{complex_raw_query} UNION #{complex_raw_query}) AS users")

Brute force it:

first_name_relation = User.where(:first_name => 'Tobias') # ActiveRecord::Relation
last_name_relation  = User.where(:last_name  => 'Fünke') # ActiveRecord::Relation

all_name_relations = User.none
first_name_relation.each do |ar|
  all_name_relations.new(ar)
end
last_name_relation.each do |ar|
  all_name_relations.new(ar)
end

Hopefully this is useful to someone out there -- you can make a second query to find the matches by id:

ids = last_name_relation.ids + first_name_relation.ids
User.where(id: ids)

I realize this may not be the most efficient with 3 database requests, but it gets the job done and is simple to understand.