From 546de0f6c6a91e4e2019f4759098caa5185176e0 Mon Sep 17 00:00:00 2001
From: Ulysse Buonomo <buonomo.ulysse@gmail.com>
Date: Fri, 24 Sep 2021 09:46:54 +0200
Subject: [PATCH] Avoid eager loading in Relation#pretty_print

We mimic the behaviour used with `#inspect`, only fetching up to 11
elements if records are not loaded already.

Here's the reproduction:

```
require "activerecord"

ActiveRecord::Base.establish_connection YOUR_DB_URL_THERE

ActiveRecord::Migration.new.tap do |m|
  m.create_table :foos do |t|
    t.text :name
  end
end

class Foo < ActiveRecord::Base
end

100.times { Foo.create(name: "foo#{_1}") }

pp Foo.all # loads the whole table.
```

Signed-off-by: Ulysse Buonomo <buonomo.ulysse@gmail.com>
---
 activerecord/CHANGELOG.md                  | 12 +++++++++
 activerecord/lib/active_record/relation.rb |  9 +++++--
 activerecord/test/cases/relations_test.rb  | 29 ++++++++++++++++++++++
 3 files changed, 48 insertions(+), 2 deletions(-)

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index dcad1820c5..0da3b5fd94 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,15 @@
+*   Avoid loading every records in `ActiveRecord::Relation#pretty_print`
+
+    ```ruby
+    # Before
+    pp Foo.all # Loads the whole table.
+
+    # After
+    pp Foo.all # Shows 10 items and an ellipsis.
+    ```
+
+    *Ulysse Buonomo*
+
 *   Change `QueryMethods#in_order_of` to drop records not listed in values.
 
     `in_order_of` now filters down to the values provided, to match the behavior of the `Enumerable` version.
diff --git a/activerecord/lib/active_record/relation.rb b/activerecord/lib/active_record/relation.rb
index 92082306c0..6d91b64e46 100644
--- a/activerecord/lib/active_record/relation.rb
+++ b/activerecord/lib/active_record/relation.rb
@@ -773,8 +773,13 @@ module ActiveRecord
       end
     end
 
-    def pretty_print(q)
-      q.pp(records)
+    def pretty_print(pp)
+      subject = loaded? ? records : annotate("loading for pp")
+      entries = subject.take([limit_value, 11].compact.min)
+
+      entries[10] = "..." if entries.size == 11
+
+      pp.pp(entries)
     end
 
     # Returns true if relation is blank.
diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index 6e1bb26cff..75fa17b9c4 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -2021,6 +2021,35 @@ class RelationTest < ActiveRecord::TestCase
     end
   end
 
+  test "relations limit the records in #pretty_print at 10" do
+    relation = Post.limit(11)
+    out = StringIO.new
+    PP.pp(relation, out)
+    assert_equal 10, out.string.scan(/#<\w*Post:/).size
+    assert out.string.end_with?("\"...\"]\n"), "Did not end with an ellipsis."
+  end
+
+  test "relations don't load all records in #pretty_print" do
+    assert_sql(/LIMIT|ROWNUM <=|FETCH FIRST/) do
+      PP.pp Post.all, StringIO.new # avoid outputting.
+    end
+  end
+
+  test "loading query is annotated in #pretty_print" do
+    assert_sql(%r(/\* loading for pp \*/)) do
+      PP.pp Post.all, StringIO.new # avoid outputting.
+    end
+  end
+
+  test "already-loaded relations don't perform a new query in #pretty_print" do
+    relation = Post.limit(2)
+    relation.to_a
+
+    assert_no_queries do
+      PP.pp relation, StringIO.new # avoid outputting.
+    end
+  end
+
   test "using a custom table affects the wheres" do
     post = posts(:welcome)