MONGOID 5800 When initializing an embedded association with an invalid hash, got undefined method `with_indifferent_access' error

[kyleburgess2025]

Currently, if a document with an embeds_many relationship is created with the embedded field set to a single hash, instead of an array of hashes, an error is raised with undefined method 'with_indifferent_access'. This behavior is new in 8.0. This PR allows backwards compatibility by checking if a value has the method with_indifferent_access, and, if not, we can assume it is an array that should be a hash. We convert it to a hash and call with_indifferent_access on the result.

[Copilot]

Pull request overview

This PR addresses a regression in Mongoid 8.0 where passing a single hash instead of an array of hashes to an embeds_many association raises an undefined method 'with_indifferent_access' error. The fix adds backwards compatibility by detecting when an array element doesn't respond to with_indifferent_access and converting it to a hash before processing.

Key changes:

• Added fallback logic to handle array elements that need conversion to hashes
• Introduced test coverage for the edge case of passing a hash to an embeds_many association

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
spec/integration/associations/embeds_many_spec.rb	Added test cases verifying that passing a hash instead of an array to an embeds_many association doesn't raise errors
lib/mongoid/association/nested/many.rb	Added conditional logic to convert non-hash-like array elements to hashes before calling with_indifferent_access

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Using Hash[*attrs] assumes attrs is an array with an even number of elements (key-value pairs). If attrs has an odd number of elements, this will raise an ArgumentError. Add validation or rescue logic to handle malformed input gracefully.

Suggested change

	process_attributes(parent, Hash[*attrs].with_indifferent_access)
	if attrs.respond_to?(:to_a) && attrs.to_a.length.even?
	process_attributes(parent, Hash[*attrs].with_indifferent_access)
	else
	# Malformed input: skip or handle as needed
	# Optionally, log a warning here
	next
	end

[kyleburgess2025]

do we need more guards here in case it can't be cast to a hash?

[jamis]

I think it's probably smart to check that array has an even number of elements, before creating a hash from it. I think an empty Hash is okay here -- I can imagine someone doing Person.new(children: {}) (thus creating a new person with a single child).

We should probably also raise an exception in the final else clause, so that we never have data being silently ignored. A simple ArgumentError would probably suffice, indicating that the arguments aren't supported or something.

[kyleburgess2025]

agreed - I didn't add any extra testing for this, I can't think of any scenarios where an uneven number of fields would be provided, but I agree that it's a good thing to check for.

[jamis]

This is a good solution, handling an edge case that we hadn't considered. As the OP noted in the ticket, this is technically not valid data, but if someone ran into this, and if we can handle the situation gracefully, we might as well.

[jamis]

I think it's probably smart to check that array has an even number of elements, before creating a hash from it. I think an empty Hash is okay here -- I can imagine someone doing Person.new(children: {}) (thus creating a new person with a single child).

We should probably also raise an exception in the final else clause, so that we never have data being silently ignored. A simple ArgumentError would probably suffice, indicating that the arguments aren't supported or something.