prepend table name for `Relation#select` columns.

This fixes a bug where `select(:id)` combined with `joins()` raised:

```
ActiveRecord::StatementInvalid: SQLite3::SQLException: ambiguous column name: id:
SELECT  id, authors.author_address_id
FROM "posts"
INNER JOIN "authors"
ON "authors"."id" = "posts"."author_id"
ORDER BY posts.id LIMIT 3
```

The `select_values` are still String and Symbols because other parts (mainly calculations.rb)
rely on that fact.

/cc @tenderlove

3 files changed, 20 insertions, 1 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 25a61b3614..9b2e9c4660 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,13 @@
+*   Prepend table name for column names passed to `Relation#select`.
+
+    Example:
+
+        Post.select(:id)
+        # Before: => SELECT id FROM "posts"
+        # After: => SELECT "posts"."id" FROM "posts"
+
+    *Yves Senn*
+
 *   Fail early with "Primary key not included in the custom select clause"
     in `find_in_batches`.
 
diff --git a/activerecord/lib/active_record/relation/query_methods.rb b/activerecord/lib/active_record/relation/query_methods.rb
index 979216bee7..d392f759bd 100644
--- a/activerecord/lib/active_record/relation/query_methods.rb
+++ b/activerecord/lib/active_record/relation/query_methods.rb
@@ -987,7 +987,10 @@ module ActiveRecord
 
     def build_select(arel, selects)
       if !selects.empty?
-        arel.project(*selects)
+        expanded_select = selects.map do |field|
+          columns_hash.key?(field.to_s) ? arel_table[field] : field
+        end
+        arel.project(*expanded_select)
       elsif from_value
         arel.project(Arel.star)
       else
diff --git a/activerecord/test/cases/relations_test.rb b/activerecord/test/cases/relations_test.rb
index 9227c2b72f..e874c93110 100644
--- a/activerecord/test/cases/relations_test.rb
+++ b/activerecord/test/cases/relations_test.rb
@@ -1505,6 +1505,12 @@ class RelationTest < ActiveRecord::TestCase
     end
   end
 
+  test "joins with select" do
+    posts = Post.joins(:author).select("id", "authors.author_address_id").order("posts.id").limit(3)
+    assert_equal [1, 2, 4], posts.map(&:id)
+    assert_equal [1, 1, 1], posts.map(&:author_address_id)
+  end
+
   test "delegations do not leak to other classes" do
     Topic.all.by_lifo
     assert Topic.all.class.method_defined?(:by_lifo)