perf: reduce DB calls made in get_fetch_values (#17671)

* perf: reduce DB calls made in `get_fetch_values`

* fix: ensure return value is same as before

* test: add test for `frappe.model.utils.get_fetch_values`

frappe/model/utils/__init__.py

@@ -90,15 +90,42 @@ def get_fetch_values(doctype, fieldname, value):
 	:param fieldname: Link fieldname selected
 	:param value: Value selected
 	"""
-	out = {}
-	meta = frappe.get_meta(doctype)
-	link_df = meta.get_field(fieldname)
-	for df in meta.get_fields_to_fetch(fieldname):
-		# example shipping_address.gistin
-		link_field, source_fieldname = df.fetch_from.split(".", 1)
-		out[df.fieldname] = frappe.db.get_value(link_df.options, value, source_fieldname)
 
-	return out
+	result = frappe._dict()
+	meta = frappe.get_meta(doctype)
+
+	# fieldname in target doctype: fieldname in source doctype
+	fields_to_fetch = {
+		df.fieldname: df.fetch_from.split(".", 1)[1] for df in meta.get_fields_to_fetch(fieldname)
+	}
+
+	# nothing to fetch
+	if not fields_to_fetch:
+		return result
+
+	# initialise empty values for target fields
+	for target_fieldname in fields_to_fetch:
+		result[target_fieldname] = None
+
+	# fetch only if Link field has a truthy value
+	if not value:
+		return result
+
+	db_values = frappe.db.get_value(
+		meta.get_options(fieldname),  # source doctype
+		value,
+		tuple(set(fields_to_fetch.values())),  # unique source fieldnames
+		as_dict=True,
+	)
+
+	# if value doesn't exist in source doctype, get_value returns None
+	if not db_values:
+		return result
+
+	for target_fieldname, source_fieldname in fields_to_fetch.items():
+		result[target_fieldname] = db_values.get(source_fieldname)
+
+	return result
 
 
 @site_cache(maxsize=128)

frappe/tests/test_model_utils.py

@@ -0,0 +1,28 @@
+import unittest
+
+import frappe
+from frappe.model.utils import get_fetch_values
+
+
+class TestModelUtils(unittest.TestCase):
+	def test_get_fetch_values(self):
+		doctype = "ToDo"
+
+		# no fields to fetch
+		self.assertEqual(get_fetch_values(doctype, "role", "System Manager"), {})
+
+		# no value
+		self.assertEqual(get_fetch_values(doctype, "assigned_by", None), {"assigned_by_full_name": None})
+
+		# no db values
+		self.assertEqual(
+			get_fetch_values(doctype, "assigned_by", "~not-a-user~"), {"assigned_by_full_name": None}
+		)
+
+		# valid db values
+		user = "test@example.com"
+		full_name = frappe.db.get_value("User", user, "full_name")
+
+		self.assertEqual(
+			get_fetch_values(doctype, "assigned_by", user), {"assigned_by_full_name": full_name}
+		)