Add a few more notes 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).
 
-_1493 TILs and counting..._
+_1494 TILs and counting..._
 
 ---
 
@@ -980,6 +980,7 @@ _1493 TILs and counting..._
 - [Parse Request Params In Rack::Attack Block](rails/parse-request-params-in-rack-attack-block.md)
 - [Perform SQL Explain With ActiveRecord](rails/perform-sql-explain-with-activerecord.md)
 - [Polymorphic Path Helpers](rails/polymorphic-path-helpers.md)
+- [Prefer select_all Over execute For Read Queries](rails/prefer-select-all-over-execute-for-read-queries.md)
 - [Pretend Generations](rails/pretend-generations.md)
 - [Prevent Writes With A Sandboxed Rails Console](rails/prevent-writes-with-a-sandboxed-rails-console.md)
 - [Query A Single Value From The Database](rails/query-a-single-value-from-the-database.md)

rails/prefer-select-all-over-execute-for-read-queries.md

@@ -0,0 +1,63 @@
+# Prefer select_all Over execute For Read Queries
+
+Though the `#execute` function provided by ActiveRecord technically works as a
+general-purpose query runner for strings of raw SQL, it has some downsides.
+
+First, let's say we have a large semi-complex (better in SQL than ActiveRecord
+DSL) SQL query defined in a heredoc.
+
+```ruby
+books_by_status_query = <<-SQL
+  select
+    books.*,
+    latest_statuses.status as current_status,
+    array_to_json(array_agg(...)) as reading_statuses
+  from books
+  -- plus several left joins
+  -- where clause, group by, and order by
+SQL
+```
+
+I reflexively reach for
+[`#execute`](https://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/DatabaseStatements.html#method-i-execute)
+in a situation like that:
+
+```ruby
+result = ActiveRecord::Base.connection.execute(books_by_status_query)
+```
+
+However, if we're doing a read-only query and we are expecting multiple rows in
+the result, then we are better off reaching for
+[`#select_all`](https://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/DatabaseStatements.html#method-i-select_all).
+
+```ruby
+result = ActiveRecord::Base.connection.select_all(books_by_status_query)
+```
+
+It has the advantage of semantically communicating that it's just a read and
+won't have any side-effects. It also avoids an unnecessary clear of the query
+cache.
+
+> Note: [when execute is used] the query is assumed to have side effects and
+> the query cache will be cleared. If the query is read-only, consider using
+> select_all instead.
+
+The `#execute` method also has been known to leak memory with some database
+connectors.
+
+> Note: depending on your database connector, the result returned by this
+> method may be manually memory managed. Consider using exec_query wrapper
+> instead.
+
+We can then iterate through and transform the results just as we would have
+done with `#execute`.
+
+```ruby
+result.map do |row|
+  row.tap do |hash|
+    hash["reading_statuses"] = JSON.parse(hash["reading_statuses"])
+  end
+
+  OpenStruct.new(row)
+end
+```