where().first vs. find_by about rails-style-guide

[guilhermesimoes]

Actually, find_by_field is being deprecated in Rails 4.

In rails 4, two other methods were added: find_by and find_by!. The find_by method is defined like this:

def find_by(*args)
  where(*args).take
end

Now, take differs from first in regards to order. first returns the first record according to the order of the primary key while take returns whatever the database spits out first.

So while using where().take is equivalent to find_by and choosing whether to use one of the other is a matter of taste, where().first differs from find_by in a subtle and not so obvious way.

[xinspire]

The following are not equivalent:

# bad
User.where(id: id).take

# good
User.find(id)

If the id doesn't exist, the first one returns nil, while the second one throws a ActiveRecord::RecordNotFound exception.

[rob-murray]

Worth noting find_by_field(value) and where(field: value) are two different operations resulting in different SQL queries.

find_by_field(value) SQL has LIMIT 1 applied so will be much more efficient than where(field: value) - I would argue that these are not interchangeable and should be used in different circumstances.

If you expect a AR record to exist or not, ie binary decision, yes/no, use find_by_field otherwise if you expect many, irrespective of whether you want 1 or more, use where(field: value).

If you apply .first to where() then the SQL is ordered by primary key and LIMIT 1 applied which may be different to what you expect.

[bbatsov]

See https://github.com/bbatsov/rails-style-guide#find_by

[kukula]

So let's talk about style, not deprecation 😸

[kukula]

Actually, they are not deprecated.
https://github.com/rails/activerecord-deprecated_finders
'Note that find(primary_key), find_by..., and find_by...! are not deprecated.'

[guilhermesimoes]

I swear I remember reading that find_by_field was being deprecated. You're right though.

So it seems now (in Rails 4) we have 3 ways to do the same thing:

• find_by_field(value)
• find_by(field: value)
• where(field: value).take

And there's a fourth way that's subtly different:

• where(field: value).first

[bbatsov]

@guilhermesimoes I'm 100% certain find_by_field is not deprecated in Rails 4.

[guilhermesimoes]

Yes, I'm 100% certain now too. What was deprecated was using dynamic finders with conditions, like: Post.find_by_title('Rails', conditions: { author: 'admin' }), thus my confusion.

So, what's your opinion on the preferred way of finding something?

[kukula]

find_by_field(value) looks more readable for me.

[pechorin]

@kukula in case if simple predicate - yes, it's more readable. But in real world sometimes it's okay to work with cases like this:

# this
Order.find_or_create_by_user_id_and_item_id(@user.id, @item.id)

# or this
Order.where(:user => @user, :item => @item).find_or_create

So, what variant is more readable for you now? :)

[kukula]

@pechorin I'm not talking about deprecated finders. I'm talking about find_by_field(value) vs where(field: value).first

[blairanderson]

according to http://edgeguides.rubyonrails.org/4_0_release_notes.html#extraction-of-features-to-gems the majority of dynamic finders are being moved to an external gem. Here is a specific note that shows All dynamic methods except for find_by_... and find_by_...! are deprecated.

find_all_by_... can be rewritten using where(...)
find_last_by_... can be rewritten using where(...).last
scoped_by_... can be rewritten using where(...)

The preferred method is where(field: value).first

[kukula]

Great! Thank you @bbatsov !

[fffx]

@xinspire And if id is a out of range value User.find(id) will raise ActiveRecord::RecordNotfound, but User.where(id: id).take will raise ActiveModel::RangeError

see gitlab 500 https://gitlab.com/coldnight/ci-test/issues/464444444444444444444444444444444444

[lanadz-shopify]

just extra 2 cents here where(x).first adds that implicit ORDER BY primary_key ASC LIMIT 1 that might be a hit on performance, whereas find_by just adds LIMIT 1 to the query and no ordering is happening