Mapping columns using reflection
In the previous two examples, we needed to explicitly map each column with the table using class and its attributes. In this method, we do not need to provide this explicit mapping of each table column separately. Using reflection, this task is automatically done by providing the metadata object and the SQLAlchemy engine connection. In the `__table__` attribute. We can then use the engine and session objects to query the actor table to fetch the first name as done in earlier examples using the column name `first_name` as the attribute name itself.
Python3
import sqlalchemy as db from sqlalchemy.ext.declarative import declarative_base Base = declarative_base() # DEFINE THE ENGINE (CONNECTION OBJECT) engine = db.create_engine( "mysql+pymysql://root:password@localhost/sakila" ) class Actor(Base): __table__ = db.Table( "actor" , Base.metadata, autoload_with = engine) # CREATE A SESSION OBJECT TO INITIATE QUERY IN DATABASE from sqlalchemy.orm import sessionmaker Session = sessionmaker(bind = engine) session = Session() # SELECT COUNT(*) FROM Table LIMIT 1; result = session.query(Actor).first() # DISPLAY FIRST NAME OF FIRST RECORD IN ACTOR TABLE print ( "First Name (Record 1):" , result.first_name) |
Output:
First Name (Record 1): PENELOPE
SQLAlchemy – Mapping Table Columns
In this article, we will see how to map table columns using SQLAlchemy in Python.
You will need a database (MySQL, PostgreSQL, SQLite, etc) to work with. Since we are going to use MySQL in this post, we will also install a SQL connector for MySQL in Python. However, none of the code implementations changes with change in the database except for the SQL connectors.
pip install pymysql
We will use the sample sakila database from MySQL. All the examples covered in this article will make use of the actor table within the sakila database. If you do not have the sakila database and want to follow along with this article without installing it then use the SQL script present in the link mentioned below to create the required schema and actor table along with the records.
Databased Used: Sakila Actor Table Script
We will be referring to the same SQL query in each of the examples mentioned below –
SELECT first_name FROM sakila.actor LIMIT 1;
The different ways in which we can map the columns in SQLAlchemy are –
- Mapping columns directly to the attribute names
- Mapping columns distinctly from attribute names
- Mapping columns using reflection
- Mapping columns using a prefix
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