Eager load reduces the number of queries. SQLAlchemy offers eager loading functions invoked via query options which give additional instructions to the Query. These options determine how to load various attributes via the Query.options() method.

Subquery Load

We want that Customer.invoices should load eagerly. The orm.subqueryload() option gives a second SELECT statement that fully loads the collections associated with the results just loaded. The name “subquery” causes the SELECT statement to be constructed directly via the Query re-used and embedded as a subquery into a SELECT against the related table.

from sqlalchemy.orm import subqueryload
c1 = session.query(Customer).options(subqueryload(Customer.invoices)).filter_by(name = ''Govind Pant'').one()

This results in the following two SQL expressions −

SELECT customers.id 
AS customers_id, customers.name 
AS customers_name, customers.address 
AS customers_address, customers.email 
AS customers_email
FROM customers
WHERE customers.name = ?
(''Govind Pant'',)

SELECT invoices.id 
AS invoices_id, invoices.custid 
AS invoices_custid, invoices.invno 
AS invoices_invno, invoices.amount 
AS invoices_amount, anon_1.customers_id 
AS anon_1_customers_id
FROM (
   SELECT customers.id 
   AS customers_id
   FROM customers
   WHERE customers.name = ?) 
   
AS anon_1 
JOIN invoices 
ON anon_1.customers_id = invoices.custid 
ORDER BY anon_1.customers_id, invoices.id 2018-06-25 18:24:47,479 
INFO sqlalchemy.engine.base.Engine (''Govind Pant'',)

To access the data from two tables, we can use the below program −

print (c1.name, c1.address, c1.email)

for x in c1.invoices:
   print ("Invoice no : {}, Amount : {}".format(x.invno, x.amount))

The output of the above program is as follows −

Govind Pant Gulmandi Aurangabad gpant@gmail.com
Invoice no : 3, Amount : 10000
Invoice no : 4, Amount : 5000

Joined Load

The other function is called orm.joinedload(). This emits a LEFT OUTER JOIN. Lead object as well as the related object or collection is loaded in one step.

from sqlalchemy.orm import joinedload
c1 = session.query(Customer).options(joinedload(Customer.invoices)).filter_by(name=''Govind Pant'').one()

This emits following expression giving same output as above −

SELECT customers.id 
AS customers_id, customers.name 
AS customers_name, customers.address 
AS customers_address, customers.email 
AS customers_email, invoices_1.id 
AS invoices_1_id, invoices_1.custid 
AS invoices_1_custid, invoices_1.invno 
AS invoices_1_invno, invoices_1.amount 
AS invoices_1_amount

FROM customers 
LEFT OUTER JOIN invoices 
AS invoices_1 
ON customers.id = invoices_1.custid

WHERE customers.name = ? ORDER BY invoices_1.id
(''Govind Pant'',)

The OUTER JOIN resulted in two rows, but it gives one instance of Customer back. This is because Query applies a “uniquing” strategy, based on object identity, to the returned entities. Joined eager loading can be applied without affecting the query results.

The subqueryload() is more appropriate for loading related collections while joinedload() is better suited for many-to-one relationship.