Add Prefer select_all Over execute For Read Queries as a Rails TIL

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

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+# 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.
+
+> Note: 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.
+
+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
+```