introduce connection.supports_views? and basic view tests.

`AbstractAdapter#supports_views?` defaults to `false` so we have to turn it on
in adapter subclasses. Currently the flag only controls test execution.

/cc @yahonda

activerecord/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Introduce `connection.supports_views?` to check wether the current adapter
+    has support for SQL views. Connection adapters should define this method.
+
+    *Yves Senn*
+
 *   Allow included modules to override association methods.
 
     Fixes #16684.

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -240,6 +240,11 @@ def supports_foreign_keys?
         false
       end
 
+      # Does this adapter support views?
+      def supports_views?
+        false
+      end
+
       # This is meant to be implemented by the adapters that support extensions
       def disable_extension(name)
       end

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -197,6 +197,10 @@ def supports_foreign_keys?
         true
       end
 
+      def supports_views?
+        version[0] >= 5
+      end
+
       def native_database_types
         NATIVE_DATABASE_TYPES
       end
@@ -779,10 +783,6 @@ def mariadb?
         full_version =~ /mariadb/i
       end
 
-      def supports_views?
-        version[0] >= 5
-      end
-
       def supports_rename_index?
         mariadb? ? false : (version[0] == 5 && version[1] >= 7) || version[0] >= 6
       end

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -158,6 +158,10 @@ def supports_foreign_keys?
         true
       end
 
+      def supports_views?
+        true
+      end
+
       def index_algorithms
         { concurrently: 'CONCURRENTLY' }
       end

activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

@@ -183,6 +183,10 @@ def supports_add_column?
         true
       end
 
+      def supports_views?
+        true
+      end
+
       def active?
         @active != false
       end

activerecord/test/cases/view_test.rb

@@ -0,0 +1,42 @@
+require "cases/helper"
+require "models/book"
+
+if ActiveRecord::Base.connection.supports_views?
+class ViewWithPrimaryKeyTest < ActiveRecord::TestCase
+  fixtures :books
+
+  class Ebook < ActiveRecord::Base
+    self.primary_key = "id"
+  end
+
+  setup do
+    @connection = ActiveRecord::Base.connection
+    @connection.execute <<-SQL
+      CREATE VIEW ebooks
+        AS SELECT id, name, status FROM books WHERE format = 'ebook'
+    SQL
+  end
+
+  teardown do
+    @connection.execute "DROP VIEW IF EXISTS ebooks"
+  end
+
+  def test_reading
+    books = Ebook.all
+    assert_equal [books(:rfr).id], books.map(&:id)
+    assert_equal ["Ruby for Rails"], books.map(&:name)
+  end
+
+  def test_table_exists
+    skip "SQLite does not currently treat views as tables" if current_adapter?(:SQLite3Adapter)
+    view_name = Ebook.table_name
+    assert @connection.table_exists?(view_name), "'#{view_name}' table should exist"
+  end
+
+  def test_column_definitions
+    assert_equal([["id", :integer],
+                  ["name", :string],
+                  ["status", :integer]], Ebook.columns.map { |c| [c.name, c.type] })
+  end
+end
+end