Add Empty find_by Returns First Record as a Rails TIL

rails/empty-find-by-returns-first-record.md

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+# Empty find_by Returns First Record
+
+During a RubyConf 2024 talk, a speaker mentioned that if you pass `nil` to
+[ActiveRecord's `#find_by`
+method](https://api.rubyonrails.org/classes/ActiveRecord/FinderMethods.html#method-i-find_by),
+it will return the first record from the database. This is a bit unintuitive,
+so lets look at an example and then I'll show you why.
+
+```ruby
+> Book.first
+#=> #<Book:0x00000001142e4c48 id: 13, title: "The Secret History", ... >
+
+> Book.find_by(nil)
+#=> #<Book:0x00000001142ca3c0 id: 13, title: "The Secret History", ... >
+```
+
+So, that is the same object in both cases, but why?
+
+Our first hint is in the SQL that gets constructed when making that method
+call.
+
+```ruby
+Book Load (2.5ms)  SELECT "books".* FROM "books" LIMIT $1  [["LIMIT", 1]]
+```
+
+It's grabbing all books and limiting to _one_ result.
+
+Lets look at the underlying implementation of the `#find_by` method.
+
+```ruby
+# File activerecord/lib/active_record/relation/finder_methods.rb, line 111
+def find_by(arg, *args)
+  where(arg, *args).take
+end
+```
+
+Sure enough, the implementation is a `#where` followed by a `#take`. Since the
+`#where` is receiving `nil` as its `arg`, there are no conditions _filtering_
+the query. And the `#take` corresponds to the `limit 1`.
+
+Knowing that, we can understand that we will also get the first record from the
+database if we call `#find_by` with `{}`. Again, no conditions to filter on, so
+give me all books limited to one.