fix: frappe.db.field_exists

- field_exists didn't work correctly
- The call to field_exists itself was wrong

frappe/database/database.py

@@ -758,7 +758,10 @@ class Database(object):
 
 	def field_exists(self, dt, fn):
 		"""Return true of field exists."""
-		return self.sql("select name from tabDocField where fieldname=%s and parent=%s", (dt, fn))
+		return self.exists('DocField', {
+			'fieldname': fn,
+			'parent': dt
+		})
 
 	def table_exists(self, doctype):
 		"""Returns True if table for given doctype exists."""

frappe/model/base_document.py

@@ -38,8 +38,8 @@ def get_controller(doctype):
 			or ["Core", False]
 
 		if custom:
-			if frappe.db.field_exists(doctype, "is_tree"):
-				is_tree = frappe.db.get_value("DocType", doctype, ("is_tree"), cache=True)
+			if frappe.db.field_exists("DocType", "is_tree"):
+				is_tree = frappe.db.get_value("DocType", doctype, "is_tree", cache=True)
 			else:
 				is_tree = False
 			_class = NestedSet if is_tree else Document