Tuesday, September 30, 2014
On Monday, 29 September 2014 23:15:36 UTC-4, Austin York wrote:
In a todo list-style app, I have the following ActiveRecord model method:
# ...
def project_name
project.tasks.length > 0 ? "#{project.name} - #{name}" : project.name
end
endThe idea is to provide additional project information if there are one or more tasks on the project.However, when invoked regularly on views this creates performance concerns (especially with a growing data set).What is the best way to optimize this query so that it doesn't create N+1 query type issues when invoked from "each" loops in the view?
+1 what Michał said about eager-loading. One additional tricky thing: prefer `size` over `length` for associations and relations. In plain Ruby, `size`, `length` and `count` are more or less identical - but for ActiveRecord collections they have slightly different meanings:
* `length` is the most straightforward: it's ALWAYS the number of records in the collection. If the collection isn't currently loaded, calling `length` on it will trigger a SQL query to load all the records.
* `count` is the opposite: it ALWAYS runs a SQL query. It doesn't load records, it uses SQL's `COUNT()` function. It can also return things that aren't numbers; doing `Task.group(:project_id).count` will give you back a hash with `project_id`s as keys and the number of matching tasks as values.
* `size` is the middle: if the collection is loaded, it works like `length`. Otherwise it works like `count`...
--Matt Jones
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