☯️ Json::Merge

[![License: MIT][📄license-img]][📄license-ref]

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if ci_badges.map(&:color).all? { it == "green"} 👇️ send money so I can do more of this. FLOSS maintenance is now my full-time job.

I've summarized my thoughts in this blog post.

Json::Merge is a standalone Ruby module that intelligently merges two versions of a JSON or JSONC 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.

When the underlying tree-sitter JSON parser surfaces JSONC comments, json-merge now preserves them directly. The jsonc-merge gem remains available as a compatibility shim for older integrations that still depend on that gem name.

• Tree-Sitter Powered: Uses tree-sitter-json for accurate AST parsing
• JSONC-Aware: Preserves // and /* */ comments when the parser exposes them
• 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.

Gem	Version / CI	Language / Format	Parser Backend(s)	Description
tree_haver		Multi	Supported Backends: MRI C, Rust, FFI, Java, Prism, Psych, Commonmarker, Markly, Citrus, Parslet	Foundation: Cross-Ruby adapter for parsing libraries (like Faraday for HTTP)
ast-merge		Text	internal	Infrastructure: Shared base classes and merge logic for all *-merge gems
bash-merge		Bash	tree-sitter-bash (via tree_haver)	Smart merge for Bash scripts
commonmarker-merge		Markdown	Commonmarker (via tree_haver)	Smart merge for Markdown (CommonMark via comrak Rust)
dotenv-merge		Dotenv	internal	Smart merge for .env files
json-merge		JSON / JSONC	tree-sitter-json (via tree_haver)	Smart merge for JSON and JSONC files
jsonc-merge		JSONC	compatibility shim	Compatibility wrapper that depends on json-merge
markdown-merge		Markdown	Commonmarker / Markly (via tree_haver), Parslet	Foundation: Shared base for Markdown mergers with inner code block merging
markly-merge		Markdown	Markly (via tree_haver)	Smart merge for Markdown (CommonMark via cmark-gfm C)
prism-merge		Ruby	Prism (prism std lib gem)	Smart merge for Ruby source files
psych-merge		YAML	Psych (psych std lib gem)	Smart merge for YAML files
rbs-merge		RBS	tree-sitter-rbs (via tree_haver), RBS (rbs std lib gem)	Smart merge for Ruby type signatures
toml-merge		TOML	Parslet + toml, Citrus + toml-rb, tree-sitter-toml (all via tree_haver)	Smart merge for TOML files

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 4
Works with MRI Ruby 3
Support & Community
Source
Documentation
Compliance	[![License: MIT][📄license-img]][📄license-ref]
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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Alternatively:

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; key 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.

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

While kettle-rb tools are free software and will always be, the project would benefit immensely from some funding. Raising a monthly budget of... "dollars" would make the project more sustainable.

We welcome both individual and corporate sponsors! We also offer a wide array of funding channels to account for your preferences (although currently Open Collective is our preferred funding platform).

If you're working in a company that's making significant use of kettle-rb tools we'd appreciate it if you suggest to your company to become a kettle-rb sponsor.

You can support the development of kettle-rb tools via GitHub Sponsors, Liberapay, PayPal, Open Collective and Tidelift.

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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 .

Made with contributors-img.

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", "~> 2.0")

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 under the following license: AGPL-3.0-only. See LICENSE.md for details.

If none of the available licenses suit your use case, please contact us to discuss a custom commercial license.

See LICENSE.md for the official copyright notice.

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.

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💌 💌 💌

Thanks for RTFM. ☺️