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Deep Merge is a simple set of utility functions for Hash. It permits you to merge elements inside a hash together recursively. The manner by which it does this is somewhat arbitrary (since there is no defining standard for this) but it should end up being pretty intuitive and do what you expect.

You can learn a lot more about this by reading the test file. It's pretty well documented and has many examples of various merges from very simple to pretty complex.

The primary need that caused me to write this library is the merging of elements coming from HTTP parameters and related stored parameters in session. This lets a user build up a set of parameters over time, modifying individual items.

DeepMerge Core Documentation

The deep_merge! method permits merging of arbitrary child elements. The two top level elements must be hashes. These hashes can contain unlimited (to stack limit) levels of child elements. These child elements do not have to be of the same type. Where child elements are of the same type, deep_merge will attempt to merge them together. Where child elements are not of the same type, deep_merge will skip or optionally overwrite the destination element with the contents of the source element at that level. For example,

source = {:x => [4, 5, '6'], :y => 2}
dest   = {:x => [1, 2, 3],   :y => [7, 8, 9]}
# => {:x => [1, 2, 3, 4, 5, '6'], :y => 2}

By default, deep_merge! will overwrite any unmergeables and merge everything else. To avoid this, use deep_merge (no bang/exclamation mark).


Options are specified in the last parameter passed, which should be in hash format

hash.deep_merge!({:x => [1, 2]}, {:knockout_prefix => '--'})
Option Default Description
:preserve_unmergeables false Set to true to skip any unmergeable elements from source
:knockout_prefix nil Set to string value to signify prefix which deletes elements from existing element
:overwrite_arrays false Set to true if you want to avoid merging arrays
:unpack_arrays nil Set to string value to run Array.join then String.split against all arrays
:sort_merged_arrays false Set to true to sort all arrays that are merged together
:merge_hash_arrays false Set to true to merge hashes within arrays
:extend_existing_arrays false Set to true to extend existing arrays, instead of overwriting them
:keep_array_duplicates false Set to true to keep duplicate entries in arrays, instead of coalescing them
:merge_nil_values false Set to true to merge nil hash values, overwriting a possibly non-nil value
:merge_debug false Set to true to get console output of merge process for debugging


Provides a way to remove elements from an existing Hash by specifying them in a special way in the incoming hash.

source = {:x => ['--1', '3']}
dest   = {:x => ['1', '2']}
dest.deep_merge!(source, :knockout_prefix => "--")
# => {:x => ['2', '3']}

Additionally, if the knockout_prefix is passed alone as a string, it will cause the entire element to be removed.

source = {:x => '--'}
dest   = {:x => [1, 2, 3]}
dest.deep_merge!(source, :knockout_prefix => "--")
# => {:x => ""}

Note that the method ko_deep_merge! defaults the knockout prefix to "--" for convenience.

source = {:x => ['--1', '3']}
dest   = {:x => ['1', '2']}
# => {:x => ['2', '3']}


Provides a way to replace Arrays instead of having them merge together.

source = {:x => ['1', '2']}
dest   = {:x => ['3', '4']}
dest.deep_merge!(source, :overwrite_arrays => true)
# => {:x => ['1', '2']}


Permits compound elements to be passed in as strings and to be converted into discrete array elements.

source = {:x => ['5', '6', '7,8']}
dest   = {:x => ['1,2,3', '4']}
dest.deep_merge!(source, :unpack_arrays => ',')
# => {:x => ['1', '2', '3', '4', '5', '6', '7', '8'}

The original purpose for this option is when receiving data from an HTML form, this makes it easy for a checkbox to pass multiple values from within a single HTML element.


Merges hashes within arrays.

source = {:x => [{:z => 2}]}
dest   = {:x => [{:y => 1}]}
dest.deep_merge!(source, :merge_hash_arrays => true)
# => {:x => [{:y => 1, :z => 2}]}


Adds source elements to existing arrays, instead of overwriting them.

source = {:x => "4" }
dest   = {:x => ["1", "2", "3"]}
dest.deep_merge!(source, :extend_existing_arrays => true)
# => {:x => ["1", "2", "3", "4"]}


Keeps duplicate entries in arrays, instead of coalescing them.

By default, without this option, duplicate entries are coalesced.

source = {:x => ["2", "3"]}
dest   = {:x => ["1", "2"]}
# => {:x => ["1", "2", "3"]}

With this option they are kept.

source = {:x => ["2", "3"]}
dest   = {:x => ["1", "2"]}
dest.deep_merge!(source, :keep_array_duplicates => true)
# => {:x => ["1", "2", "2", "3"]}


Allows nil hash values to be merged.

By default, without this option, nil hash values in the source are ignored.

source = {:x => nil}
dest   = {:x => "1"}
# => {:x => "1"}

With this option, nil values will overwrite existing values.

source = {:x => nil}
dest   = {:x => "1"}
dest.deep_merge!(source, :merge_nil_values => true)
# => {:x => nil}

Using deep_merge in Rails

To avoid conflict with ActiveSupport, load deep_merge via:

require 'deep_merge/rails_compat'

In a Gemfile:

gem "deep_merge", :require => 'deep_merge/rails_compat'

The deep_merge methods will then be defined as



deep_merge was written by Steve Midgley, and is now maintained by Daniel DeLeo. The official home of deep_merge on the internet is now


Copyright (c) 2008-2016 Steve Midgley, released under the MIT license


Recursive Merging for Ruby Hashes







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