Rails makes it really easy to do something 10 different ways to get the same result. Unfortunately, most of which aren't the most performant way. In my 10 years of building Rails apps of all different sizes, and seeing some very mature apps in production, this is the most common culprit I've seen.
I currently work on a rather large Rails app for a site that most of us here use. A common pattern for our performance pitfalls are things like this:
Tag.all.map(&:name)
versus
Tag.all.pluck(:name)
Using `#map` will instantiate a `Tag` object and do all the slow(er) metaprogramming to get you the syntactic sugar that makes interacting with an ActiveRecord object a treat. It does this, and then you only hit `#name`, completely wasting all that effort.
`#pluck` will change the SQL from `select * from tags` to `select tags.name from tags` and never instantiate a `Tag` object, instead short-circuiting and directly fetching the resulting SQL rows — which comes back as an array. It's something along the lines of:
ActiveRecord::Base.connection.exec_query(Tag.select(:id).to_sql)
Another one I see:
ProgrammingLanguage.where(tag_id: @user.tags.select(&:language_tag?).map(&:id))
versus
ProgrammingLanguage.where(tag_id: @user.tags.select(:id).where(type: 'LanguageTag'))
The first example loops over the loaded `@user.tags`, loads them if they're not already `#loaded?`, selects ones that are `type == 'LanguageTag'`, only to grab the `#id`.
The second example joins the two resulting SQL statements and calls `#to_sql` on the second statement, building one query from two queries.
Are these times when the first example would be preferred? Yeah, plenty! If your result is already `#loaded?`, then you probably don't need to hit the database again. But for this example and the ones I'm stumbling across while getting our company up-to-speed on "good ways", these are the the commonalities.
Save for only very recently, the company I work for hasn't put emphasis on real-world Ruby/Rails skills, instead "if you can code at a high level for any language, we think you can make reasonable contributions to our codebase." This has lead to hiring skilled developers that just don't know that there's a more preferred way of doing things in Rails for different contexts.
Double-edged sword, really.