🚀 Understanding reverse_merge in Ruby on Rails

November 26, 2025

When working with Ruby hashes, we often combine user input with default values. But merge isn’t always ideal — it overwrites existing keys.

That’s where reverse_merge comes in.

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What Is reverse_merge?

Unlike Ruby’s merge, Rails’ reverse_merge keeps the original values and only fills in missing ones.

{ a: 1 }.reverse_merge(a: 2)
# => { a: 1 }

Perfect for setting defaults without overwriting what’s already there.

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Why It Matters

reverse_merge helps you:

• define clean default configurations
• avoid repetitive conditionals
• make helpers/components more expressive
• keep your code intention clear

Example:

options = params.reverse_merge(page: 1, per_page: 10)

If the value exists, it stays. If not, the default applies. Simple. Explicit. Readable.

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A Helpful Mental Model

Think of reverse_merge as:

“Use these defaults unless the caller already provided a value.”

It’s a small method, but it fits beautifully with Rails’ philosophy of clear, intention-driven code.

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If this was helpful, let me know — I’m sharing more Ruby/Rails insights soon!