We can interact with these two models in the following way:Ĭreating a one-to-one relationship between models is very similar to creating many-to-one relationships. Now we have a one-to-many relationship between User and Post. Owner = relationship(User, back_populates="posts") Posts = relationship("Post", back_populates="owner") This is equivalent to the following definition: Owner = relationship(User, backref=backref('posts', uselist=True)) Owner_id = Column(Integer, ForeignKey('user.id')) For example, the following code puts a second relationship() "posts" onto the user table by specifying a backref on Post.owner: A backref is a common shortcut to place a second relationship() onto the destination table. One common keyword argument we have used so far in relationship definitions is backref. > de_link = s.query(DepartmentEmployeeLink).filter(DepartmentEmployeeLink.extra_data = 'part-time').one()įinally, adding an IT employee using the relationship Department.employees still works, as shown in the previous article: > de_link = s.query(DepartmentEmployeeLink).join(Department).filter(Department.name = 'IT').one() Then, we can find John by querying the IT department or the DepartmentEmployeeLink model. > John_working_part_time_at_IT = DepartmentEmployeeLink(department=IT, employee=John, extra_data='part-time') > from sqlalchemy.orm import sessionmaker > engine = create_engine('sqlite:///association_tables.sqlite') For example, suppose we have an employee John who works part-time in the IT department, we can insert the string 'part-time' into the column extra_data and create an DepartmentEmployeeLink object to represent this relationship. With one more extra column and two more relationships on the DepartmentEmployeeLink association model, we can store more information and be more liberal with how we want to use it. Now let's add one more column extra_data and two more relationships department and employee.ĭepartment = relationship(Department, backref=backref("employee_assoc"))Įmployee = relationship(Employee, backref=backref("department_assoc")) Notice that the DepartmentEmployeeLink class contains two foreign key columns which are enough to model the many-to-many relationship between Department and Employee. _tablename_ = 'department_employee_link'ĭepartment_id = Column(Integer, ForeignKey('department.id'), primary_key=True)Įmployee_id = Column(Integer, ForeignKey('employee.id'), primary_key=True) Hired_on = Column(DateTime, default=func.now()) In our previous article, we created the following SQLAlchemy models:įrom sqlalchemy import Column, DateTime, String, Integer, ForeignKey, funcįrom sqlalchemy.orm import relationship, backrefįrom import declarative_base In this article, we are going to dive deeper into the association table concept and see how we can use it to further solve more complicated problems. In our previous articles, we used an association table to model many-to-many relationships between tables, such as the relationship between Department and Employee. As it stands now, I have a search activity, a dbHelper class, I will be needed cursors and as far as I know, a class that will be displaying the results? Unless if that stuff is also down under my search activity.Published: Sunday 20 th April 2014 Association Tables If anyone could provide an overview of the general concept of incorporating a search feature (one that fits my needs above) or a link to a tutorial, that would be appreciated. A lot of tutorials I have found online show how to make/update/add/delete a database and entries when the app runs, but is this necessary if I am using a DB browser? A lot of the codes have simple stuff like declaring the columns, but I am unsure if this would be needed in my case. It is my first time doing something of this magnitude so I am slowly beginning to feel overwhelmed. I've also looked at the "Searchable dictionary" example referenced in the android link. I have tried googling how to do this, I was able to stumble upon which showed me how to access the database in an android app. I have gotten all the way up to "Search your data" section, which I do not know how to proceed. My database will have 3 columns: item, category, and description. I want a search widget within the activity (not the action bar), I want to be able to search a database that I will create with a DB browser, I also want the search results to be presented in the same activity (If that is possible). A little description of what I want my app to do:
0 Comments
Leave a Reply. |