Add a caveat to the latest TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1505 TILs and counting..._
+_1506 TILs and counting..._
 
 ---
 
@@ -942,6 +942,7 @@ _1505 TILs and counting..._
 - [Demodulize A Class Name](rails/demodulize-a-class-name.md)
 - [Different Ways To Add A Foreign Key Reference](rails/different-ways-to-add-a-foreign-key-reference.md)
 - [Disambiguate Where In A Joined Relation](rails/disambiguate-where-in-a-joined-relation.md)
+- [Empty find_by Returns First Record](rails/empty-find-by-returns-first-record.md)
 - [Ensure A Rake Task Cannot Write Data](rails/ensure-a-rake-task-cannot-write-data.md)
 - [Ensure Migrations Use The Latest Schema](rails/ensure-migrations-use-the-latest-schema.md)
 - [Ensure Record Saved With after_commit Callback](rails/ensure-record-saved-with-after-commit-callback.md)

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

@@ -0,0 +1,49 @@
+# 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.
+
+One small caveat: notice how there is no `order by` clause in the above SQL
+output. This differs from `Books.first` which implicitly does an order on the
+`id` column. Though these method are likely to return the same result, the
+ordering of `#find_by` is not guaranteed to be the same without an `order by`
+clause.