How to merge multiple hashes in Ruby? - ruby

Question

h = { a: 1 }
h2 = { b: 2 }
h3 = { c: 3 }
Hash#merge works for 2 hashes: h.merge(h2)
How to merge 3 hashes?
h.merge(h2).merge(h3) works but is there a better way?

Answer 1

You could do it like this:
h, h2, h3 = { a: 1 }, { b: 2 }, { c: 3 }
a = [h, h2, h3]
p Hash[*a.map(&:to_a).flatten] #= > {:a=>1, :b=>2, :c=>3}
Edit: This is probably the correct way to do it if you have many hashes:
a.inject{|tot, new| tot.merge(new)}
# or just
a.inject(&:merge)

Answer 2

Since Ruby 2.0 on that can be accomplished more graciously:
h.merge **h1, **h2
And in case of overlapping keys - the latter ones, of course, take precedence:
h = {}
h1 = { a: 1, b: 2 }
h2 = { a: 0, c: 3 }
h.merge **h1, **h2
# => {:a=>0, :b=>2, :c=>3}
h.merge **h2, **h1
# => {:a=>1, :c=>3, :b=>2}

Answer 3

You can just do
[*h,*h2,*h3].to_h
# => {:a=>1, :b=>2, :c=>3}
This works whether or not the keys are Symbols.

Answer 4

Ruby 2.6 allows merge to take multiple arguments:
h = { a: 1 }
h2 = { b: 2 }
h3 = { c: 3 }
h4 = { 'c' => 4 }
h5 = {}
h.merge(h2, h3, h4, h5) # => {:a=>1, :b=>2, :c=>3, "c"=>4}
This works with Hash.merge! and Hash.update too. Docs for this here.
Also takes empty hashes and keys as symbols or strings.
Much simpler :)

Answer 5

Answer using reduce (same as inject)
hash_arr = [{foo: "bar"}, {foo2: "bar2"}, {foo2: "bar2b", foo3: "bar3"}]
hash_arr.reduce { |acc, h| (acc || {}).merge h }
# => {:foo2=>"bar2", :foo3=>"bar3", :foo=>"bar"}
Explanation
For those beginning with Ruby or functional programming, I hope this brief explanation might help understand what's happening here.
The reduce method when called on an Array object (hash_arr) will iterate through each element of the array with the returned value of the block being stored in an accumulator (acc). Effectively, the h parameter of my block will take on the value of each hash in the array, and the acc parameter will take on the value that is returned by the block through each iteration.
We use (acc || {}) to handle the initial condition where acc is nil. Note that the merge method gives priority to keys/values in the original hash. This is why the value of "bar2b" doesn't appear in my final hash.
Hope that helps!

Answer 6

To build upon #Oleg Afanasyev's answer, you can also do this neat trick:
h = { a: 1 }
h2 = { b: 2 }
h3 = { c: 3 }
z = { **h, **h2, **h3 } # => {:a=>1, :b=>2, :c=>3}
Cheers!

Answer 7

class Hash
def multi_merge(*args)
args.unshift(self)
args.inject { |accum, ele| accum.merge(ele) }
end
end
That should do it. You could easily monkeypatch that into Hash as I have shown.

Answer 8

newHash = [h, h2, h3].each_with_object({}) { |oh, nh| nh.merge!(oh)}
# => {:a=>1, :b=>2, :c=>3}

Answer 9

Here are the 2 monkeypatched ::Hash instance methods we use in our app. Backed by Minitest specs. They use merge! instead of merge internally, for performance reasons.
class ::Hash
# Merges multiple Hashes together. Similar to JS Object.assign.
# Returns merged hash without modifying the receiver.
#
# #param *other_hashes [Hash]
#
# #return [Hash]
def merge_multiple(*other_hashes)
other_hashes.each_with_object(self.dup) do |other_hash, new_hash|
new_hash.merge!(other_hash)
end
end
# Merges multiple Hashes together. Similar to JS Object.assign.
# Modifies the receiving hash.
# Returns self.
#
# #param *other_hashes [Hash]
#
# #return [Hash]
def merge_multiple!(*other_hashes)
other_hashes.each(&method(:merge!))
self
end
end
Tests:
describe "#merge_multiple and #merge_multiple!" do
let(:hash1) {{
:a => "a",
:b => "b"
}}
let(:hash2) {{
:b => "y",
:c => "c"
}}
let(:hash3) {{
:d => "d"
}}
let(:merged) {{
:a => "a",
:b => "y",
:c => "c",
:d => "d"
}}
describe "#merge_multiple" do
subject { hash1.merge_multiple(hash2, hash3) }
it "should merge three hashes properly" do
assert_equal(merged, subject)
end
it "shouldn't modify the receiver" do
refute_changes(->{ hash1 }) do
subject
end
end
end
describe "#merge_multiple!" do
subject { hash1.merge_multiple!(hash2, hash3) }
it "should merge three hashes properly" do
assert_equal(merged, subject)
end
it "shouldn't modify the receiver" do
assert_changes(->{ hash1 }, :to => merged) do
subject
end
end
end
end

Answer 10

Just for fun, you can do it also this way:
a = { a: 1 }, { b: 2 }, { c: 3 }
{}.tap { |h| a.each &h.method( :update ) }
#=> {:a=>1, :b=>2, :c=>3}

Answer 11

With modern Ruby, you wont even have to use merge unless you need to change the variable in place using the ! variant, you can just double splat (**) your way through.
h = { a: 1 }
h2 = { b: 2 }
h3 = { c: 3 }
merged_hash = { **h, **h2, **h3 }
=> { a: 1, b: 2, c:3 }