TLDR

• When counting records in an association, you should use #size in most cases.
• It will not work if the parent record has never been saved. Also there are finer distinctions between #size and #count. See below.

count

• Always makes a COUNT(*) query if a counter cache is not set up.
• If a counter cache is set up on the association, #count will return that cached value instead of executing a new query.

size, if the association has already been loaded

• Counts elements in the already loaded array.
• Does not make another query.

size, if the association has not yet been loaded

• Makes a COUNT(*) query, just like #count

length

• Always loads the contents of the association into memory, then returns the number of elements loaded.

Caveat for unsaved records

• If you trigger a COUNT(*) by whatever means, Rails will try to load all children where the foreign key IS NULL. This is not what you want, ever.
• If you want to count children and not fall into this trap, use association.to_a.size.

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