Add ActiveRecord Query For This Or That as a rails til

rails/active-record-query-for-this-or-that.md

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+# ActiveRecord Query For This Or That
+
+When including multiple `where` clauses on a query, we are adding more
+specificity to the resulting `ActiveRecord` relation -- it's like saying we
+want records that match this _and_ that. But what about when we want to find
+records that match this _or_ that?
+
+This is supported by `ActiveRecord` through the `or` query method.
+
+Let's say we want all books that are either unpublished _or_ are published in
+2019.
+
+```ruby
+> Book.where(status: 'unpublished').or(Book.where(publication_year: 2019))
+=> #<ActiveRecord::Relation [...]>
+```
+
+This will generate SQL that includes a `where` clause like the following:
+
+```sql
+where (books.status = 'unpublished' or books.publication_year = 2019)
+```
+
+See the
+[docs](https://api.rubyonrails.org/classes/ActiveRecord/QueryMethods.html#method-i-or)
+for more details.