Add Disambiguate Where In A Joined Relation as a rails til

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_802 TILs and counting..._
+_803 TILs and counting..._
 
 ---
 
@@ -490,6 +490,7 @@ _802 TILs and counting..._
 - [Custom Validation Message](rails/custom-validation-message.md)
 - [Delete Paranoid Records](rails/delete-paranoid-records.md)
 - [Demodulize A Class Name](rails/demodulize-a-class-name.md)
+- [Disambiguate Where In A Joined Relation](rails/disambiguate-where-in-a-joined-relation.md)
 - [Generating And Executing SQL](rails/generating-and-executing-sql.md)
 - [Hash Slicing](rails/hash-slicing.md)
 - [Ignore Poltergeist JavaScript Errors](rails/ignore-poltergeist-javascript-errors.md)

rails/disambiguate-where-in-a-joined-relation.md

@@ -0,0 +1,29 @@
+# Disambiguate Where In A Joined Relation
+
+When you join two tables using ActiveRecord
+
+```ruby
+Post.joins(:author)
+```
+
+You get a relation that involves both of the tables. This allows you to
+write queries that work off the data in that join.
+
+The primary table in the join is `posts`. Any column references in a
+`#where` call will be assumed to be related to `posts`. If you want to
+reference a column on the `authors` table, you'll need to provide that
+specificity.
+
+The hash syntax for `#where` is a great way to do that:
+
+```ruby
+Post.joins(:author).where({ authors: { name: "John Steinbeck" }})
+```
+
+You can also use the string syntax:
+
+```ruby
+Post.joins(:author).where("authors.name = ?", "John Steinbeck")
+```
+
+[source](https://apidock.com/rails/v4.2.7/ActiveRecord/QueryMethods/where)