Rails: How to get objects with at least one child?

Question

After googling, browsing SO and reading, there doesn\'t seem to be a Rails-style way to efficiently get only those Parent objects which have at leas

Answer 1

try including the children with #includes()

Parent.includes(:children).all.reject { |parent| parent.children.empty? }

This will make 2 queries:

SELECT * FROM parents;
SELECT * FROM children WHERE parent_id IN (5, 6, 8, ...);

[UPDATE]

The above solution is usefull when you need to have the Child objects loaded. But children.empty? can also use a counter cache^1,2 to determine the amount of children.

For this to work you need to add a new column to the parents table:

# a new migration
def up
  change_table :parents do |t|
    t.integer :children_count, :default => 0
  end

  Parent.reset_column_information
  Parent.all.each do |p|
    Parent.update_counters p.id, :children_count => p.children.length
  end
end

def down
  change_table :parents do |t|
    t.remove :children_count
  end
end

Now change your Child model:

class Child
  belongs_to :parent, :counter_cache => true
end

At this point you can use size and empty? without touching the children table:

Parent.all.reject { |parent| parent.children.empty? }

Note that length doesn't use the counter cache whereas size and empty? do.