☯️ Json::Merge

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Json::Merge is a standalone Ruby module that intelligently merges two versions of a JSON file using tree-sitter AST analysis. It's like a smart "git merge" specifically designed for JSON configuration files. Built on top of ast-merge, it shares the same architecture as prism-merge for Ruby source files.

For JSONC (JSON with Comments) support, see the jsonc-merge gem.

• Tree-Sitter Powered: Uses tree-sitter-json for accurate AST parsing
• Intelligent: Matches objects and arrays by structural signatures
• Fuzzy Property Matching: ObjectMatchRefiner matches similar property names (e.g., databaseUrl ↔ database_url) using Levenshtein distance for naming convention differences
• Full Provenance: Tracks origin of every node
• Standalone: Minimal dependencies - just ast-merge and ruby_tree_sitter
• Customizable:
  • signature_generator - callable custom signature generators
  • preference - setting of :template, :destination, or a Hash for per-node-type preferences
  • node_splitter - Hash mapping node types to callables for per-node-type merge customization (see ast-merge docs)
  • add_template_only_nodes - setting to retain nodes that do not exist in destination
  • match_refiners - array of refiners for fuzzy matching (e.g., ObjectMatchRefiner)

require "json/merge"

template = File.read("template.json")
destination = File.read("destination.json")

merger = Json::Merge::SmartMerger.new(template, destination)
result = merger.merge

File.write("merged.json", result.to_json)

The *-merge gem family provides intelligent, AST-based merging for various file formats. At the foundation is tree_haver, which provides a unified cross-Ruby parsing API that works seamlessly across MRI, JRuby, and TruffleRuby.

tree_haver supports multiple parsing backends, but not all backends work on all Ruby platforms:

Legend: ✅ = Works, ❌ = Does not work, ❓ = Untested

Why some backends don't work on certain platforms:

• JRuby: Runs on the JVM; cannot load native C/Rust extensions (.so files)
• TruffleRuby: Has C API emulation via Sulong/LLVM, but it doesn't expose all MRI internals that native extensions require (e.g., RBasic.flags, rb_gc_writebarrier)
• FFI on TruffleRuby: TruffleRuby's FFI implementation doesn't support returning structs by value, which tree-sitter's C API requires

Example implementations for the gem templating use case:

Tokens to Remember
Works with JRuby
Works with Truffle Ruby
Works with MRI Ruby 3
Support & Community
Source
Documentation
Compliance
Style
Maintainer 🎖️
... 💖	🧊 🐙 🛖 🧪

Compatible with MRI Ruby 3.2.0+, and concordant releases of JRuby, and TruffleRuby.

Federated DVCS Repository	Status	Issues	PRs	Wiki	CI	Discussions
🧪 kettle-rb/json-merge on GitLab	The Truth	💚	💚	💚	🐭 Tiny Matrix	➖
🧊 kettle-rb/json-merge on CodeBerg	An Ethical Mirror (Donate)	💚	💚	➖	⭕️ No Matrix	➖
🐙 kettle-rb/json-merge on GitHub	Another Mirror	💚	💚	💚	💯 Full Matrix	💚
🎮️ Discord Server		Let's	talk	about	this	library!

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Install the gem and add to the application's Gemfile by executing:

bundle add json-merge

If bundler is not being used to manage dependencies, install the gem by executing:

gem install json-merge

This gem is cryptographically signed, and has verifiable SHA-256 and SHA-512 checksums by stone_checksums. Be sure the gem you install hasn’t been tampered with by following the instructions below.

Add my public key (if you haven’t already, expires 2045-04-29) as a trusted certificate:

gem cert --add <(curl -Ls https://raw.github.com/galtzo-floss/certs/main/pboling.pem)

You only need to do that once. Then proceed to install with:

gem install json-merge -P HighSecurity

The HighSecurity trust profile will verify signed gems, and not allow the installation of unsigned dependencies.

If you want to up your security game full-time:

bundle config set --global trust-policy MediumSecurity

MediumSecurity instead of HighSecurity is necessary if not all the gems you use are signed.

NOTE: Be prepared to track down certs for signed gems and add them the same way you added mine.

This gem requires the tree-sitter-json parser library to be installed on your system. The parser is a native shared library (.so on Linux, .dylib on macOS) that provides JSON syntax parsing capabilities. For JSONC (JSON with Comments) support, use the jsonc-merge gem instead.

Download pre-built parsers from Faveod/tree-sitter-parsers:

Linux (x64):

# Download and extract
curl -Lo parsers.tar.gz https://github.com/Faveod/tree-sitter-parsers/releases/download/v4.10/tree-sitter-parsers-4.10-linux-x64.tar.gz
tar -xzf parsers.tar.gz

# Install system-wide (requires sudo)
sudo cp libtree-sitter-json.so /usr/lib/
sudo ldconfig

# Or install locally and set environment variable
mkdir -p ~/.local/lib/tree-sitter
cp libtree-sitter-json.so ~/.local/lib/tree-sitter/
export TREE_SITTER_JSON_PATH="$HOME/.local/lib/tree-sitter/libtree-sitter-json.so"

Debian/Ubuntu (amd64):

curl -Lo parsers.deb https://github.com/Faveod/tree-sitter-parsers/releases/download/v4.10/tree-sitter-parsers-4.10-amd64.deb
sudo dpkg -i parsers.deb

macOS (Apple Silicon):

curl -Lo parsers.tar.gz https://github.com/Faveod/tree-sitter-parsers/releases/download/v4.10/tree-sitter-parsers-4.10-macos-arm64.tar.gz
tar -xzf parsers.tar.gz

# Install to a location and set environment variable
mkdir -p ~/.local/lib/tree-sitter
cp libtree-sitter-json.dylib ~/.local/lib/tree-sitter/
export TREE_SITTER_JSON_PATH="$HOME/.local/lib/tree-sitter/libtree-sitter-json.dylib"

git clone https://github.com/tree-sitter/tree-sitter-json.git
cd tree-sitter-json
make
sudo cp libtree-sitter-json.so /usr/lib/  # Linux
# or
sudo cp libtree-sitter-json.dylib /usr/local/lib/  # macOS

Some package managers provide tree-sitter parsers:

Fedora Atomic (Silverblue, Kinoite, Bazzite, Aurora, etc.):

# Install via rpm-ostree (requires reboot)
rpm-ostree install libtree-sitter-json

# The library will be installed to /usr/lib64/libtree-sitter-json.so
# You may need to set the environment variable:
export TREE_SITTER_JSON_PATH="/usr/lib64/libtree-sitter-json.so"

Fedora (traditional):

sudo dnf install libtree-sitter-json

Arch Linux:

# Check AUR for tree-sitter-json
yay -S tree-sitter-json

If the parser is not in a standard location (/usr/lib/, /usr/lib64/, /usr/local/lib/), set the TREE_SITTER_JSON_PATH environment variable to point to the parser library:

export TREE_SITTER_JSON_PATH="/path/to/libtree-sitter-json.so"

Note: Some distributions install the library with a version number suffix (e.g., libtree-sitter-json.so.14 instead of libtree-sitter-json.so). If the gem can't find the parser, check for versioned files and either:

• Set TREE_SITTER_JSON_PATH to the full versioned path, or
• Create a symlink: sudo ln -s /usr/lib64/libtree-sitter-json.so.14 /usr/lib64/libtree-sitter-json.so Add this to your shell profile (.bashrc, .zshrc, etc.) for persistence.

In addition to the tree-sitter parser library, you need a Ruby gem that provides bindings to tree-sitter. Choose one of the following based on your Ruby implementation:

gem install ruby_tree_sitter

Or add to your Gemfile:

gem "ruby_tree_sitter", "~> 2.0"

gem install ffi

Or add to your Gemfile:

gem "ffi"

The tree_haver gem (a dependency of json-merge) will automatically detect and use the appropriate backend based on which gems are available.

Note: The ruby_tree_sitter gem only compiles on MRI Ruby. For JRuby or TruffleRuby, you must use the FFI backend.

Control which version to use when nodes have matching signatures but different content:

# Use template version (for config updates)
merger = Json::Merge::SmartMerger.new(
  template,
  destination,
  preference: :template,
)

# Use destination version (default - preserve customizations)
merger = Json::Merge::SmartMerger.new(
  template,
  destination,
  preference: :destination,
)

Control whether to add nodes that only exist in the template:

# Add template-only nodes
merger = Json::Merge::SmartMerger.new(
  template,
  destination,
  add_template_only_nodes: true,
)

When JSON object properties (key-value pairs) don't match by exact key name, the ObjectMatchRefiner uses fuzzy matching to pair entries with:

• Similar key names (e.g., databaseUrl vs database_url)
• Keys with typos or different naming conventions (camelCase vs snake_case)
• Array elements with similar structure or content

# Enable object fuzzy matching
merger = Json::Merge::SmartMerger.new(
  template,
  destination,
  match_refiners: [
    Json::Merge::ObjectMatchRefiner.new(threshold: 0.5),
  ],
)

# Custom weights for key-centric matching
refiner = Json::Merge::ObjectMatchRefiner.new(
  threshold: 0.6,
  key_weight: 0.8,   # Focus more on key names
  value_weight: 0.2,  # Less focus on values
)

Enable debug logging to see merge decisions:

export JSON_MERGE_DEBUG=1

require "json/merge"

template_content = File.read("template.json")
dest_content = File.read("destination.json")

merger = Json::Merge::SmartMerger.new(template_content, dest_content)
result = merger.merge

File.write("merged.json", result.to_json)

require "json/merge"

source = File.read("config.json")
analysis = Json::Merge::FileAnalysis.new(source)

# Iterate over all top-level nodes
analysis.statements.each do |node|
  sig = analysis.generate_signature(node)
  puts "#{node.class}: #{sig.inspect}"
end

When property names differ between template and destination (e.g., naming convention changes), use the ObjectMatchRefiner:

require "json/merge"

template = <<~JSON
  {
    "databaseUrl": "postgres://localhost/app",
    "cacheTimeout": 3600,
    "apiEndpoint": "https://api.example.com"
  }
JSON

destination = <<~JSON
  {
    "database_url": "postgres://localhost/custom",
    "cache_ttl": 7200,
    "api_endpoint": "https://custom.example.com"
  }
JSON

# Default merge won't match keys (names differ - camelCase vs snake_case)
# Use ObjectMatchRefiner for fuzzy matching
merger = Json::Merge::SmartMerger.new(
  template,
  destination,
  match_refiners: [
    Json::Merge::ObjectMatchRefiner.new(threshold: 0.5),
  ],
)
result = merger.merge

# Properties are matched despite naming convention differences:
# - databaseUrl ↔ database_url (similar when normalized)
# - cacheTimeout ↔ cache_ttl (similar: "cache")
# - apiEndpoint ↔ api_endpoint (similar when normalized)

The ObjectMatchRefiner also handles array elements with similar structure:

template = <<~JSON
  {
    "users": [
      { "id": 1, "userName": "alice" },
      { "id": 2, "userName": "bob" }
    ]
  }
JSON

destination = <<~JSON
  {
    "users": [
      { "id": 1, "user_name": "alice_custom" },
      { "id": 3, "user_name": "charlie" }
    ]
  }
JSON

merger = Json::Merge::SmartMerger.new(
  template,
  destination,
  match_refiners: [
    Json::Merge::ObjectMatchRefiner.new(threshold: 0.5),
  ],
)
# Array elements with matching IDs or similar structure are paired

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I’m driven by a passion to foster a thriving open-source community – a space where people can tackle complex problems, no matter how small. Revitalizing libraries that have fallen into disrepair, and building new libraries focused on solving real-world challenges, are my passions. I was recently affected by layoffs, and the tech jobs market is unwelcoming. I’m reaching out here because your support would significantly aid my efforts to provide for my family, and my farm (11 🐔 chickens, 2 🐶 dogs, 3 🐰 rabbits, 8 🐈‍ cats).

If you work at a company that uses my work, please encourage them to support me as a corporate sponsor. My work on gems you use might show up in bundle fund.

I’m developing a new library, floss_funding, designed to empower open-source developers like myself to get paid for the work we do, in a sustainable way. Please give it a look.

Floss-Funding.dev: 👉️ No network calls. 👉️ No tracking. 👉️ No oversight. 👉️ Minimal crypto hashing. 💡 Easily disabled nags

See SECURITY.md.

If you need some ideas of where to help, you could work on adding more code coverage, or if it is already 💯 (see below) check reek, issues, or PRs, or use the gem and think about how it could be better.

We so if you make changes, remember to update it.

See CONTRIBUTING.md for more detailed instructions.

See CONTRIBUTING.md.

Everyone interacting with this project's codebases, issue trackers, chat rooms and mailing lists agrees to follow the .

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Also see GitLab Contributors: https://gitlab.com/kettle-rb/json-merge/-/graphs/main

This Library adheres to . Violations of this scheme should be reported as bugs. Specifically, if a minor or patch version is released that breaks backward compatibility, a new version should be immediately released that restores compatibility. Breaking changes to the public API will only be introduced with new major versions.

dropping support for a platform is both obviously and objectively a breaking change
—Jordan Harband (@ljharb, maintainer of SemVer) in SemVer issue 716

I understand that policy doesn't work universally ("exceptions to every rule!"), but it is the policy here. As such, in many cases it is good to specify a dependency on this library using the Pessimistic Version Constraint with two digits of precision.

For example:

spec.add_dependency("json-merge", "~> 1.0")

SemVer should, IMO, but doesn't explicitly, say that dropping support for specific Platforms is a breaking change to an API, and for that reason the bike shedding is endless.

To get a better understanding of how SemVer is intended to work over a project's lifetime, read this article from the creator of SemVer:

• "Major Version Numbers are Not Sacred"

See CHANGELOG.md for a list of releases.

The gem is available as open source under the terms of the MIT License . See LICENSE.txt for the official Copyright Notice.

• Copyright (c) 2025 Peter H. Boling, of Galtzo.com , and json-merge contributors.

Maintainers have teeth and need to pay their dentists. After getting laid off in an RIF in March, and encountering difficulty finding a new one, I began spending most of my time building open source tools. I'm hoping to be able to pay for my kids' health insurance this month, so if you value the work I am doing, I need your support. Please consider sponsoring me or the project.

To join the community or get help 👇️ Join the Discord.

To say "thanks!" ☝️ Join the Discord or 👇️ send money.

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Thanks for RTFM. ☺️