first_or_create vs find_or_create_by

问题

first_or_create seems much less documented, so I wondered if it was just because the two methods are synonymous.

回答1:

Basically:

Foo.where(attributes).first_or_create

Is the same as:

Foo.find_or_create_by(attributes)

---

#first_or_create is sometimes misunderstood as people expect it to search by the attributes given, but that is not the case. Aka

Foo.first_or_create(attributes)

Will not search for a foo that satisfies the attributes. It will take the first foo if any are present. It's useful if the conditions for finding are a subset of those used for creation. Aka

Foo.where(something: value).first_or_create(attributes)

Will find the first foo where something: value. If none is present, it will use attributes to create it.

回答2:

@ndnenkov's answer is right on but I wanted to expand on why I think one might use find_or_create:

Consider the case when Foo must have either attribute_subset_1 or attribute_subset_2 before it can be created. One could (pseudo)code this as:

if attribute_subset_1 then
  Foo.where(attribute_subset_1).first_or_create(attributes)
elsif attribute_subset_2 then
  Foo.where(attribute_subset_2).first_or_create(attributes)
else
  Raise error " insufficient attributes"
end

This is a case where I think find_or_create_by does not work...at least not easily. I would be curious to hear if anything sees this differently but for me I found this use of first_or_create perfect for my needs.

来源：https://stackoverflow.com/questions/42178674/first-or-create-vs-find-or-create-by