## Database Persistence ##
The persistence package includes classes to store different class objects in a Postgres database.

### models ###
This defines models for all class objects that we want to persist. It is used for Object Relation Mapping (ORM)
of the class objects to database table columns

### repositories ###
This defines repository classes that contain CRUD methods for database operations. The constructor of all repositories requires
The database name to connect to and the application environment (PROD or TEST). Tests use a different database
from the production environment, which is why this is necessary. An example is shown below
```python
from persistence import CityRepo
# instantiate city repo for hub production database
city_repo = CityRepo(db_name='hub', app_env='PROD')
```
All database operations are conducted with the production database (*PROD*) named *hub* in the example above

### config_db ##
This Python file is a configuration class that contains variables that map to configuration parameters in a .env file. 
It also contains a method ``def conn_string()`` which returns the connection string to a Postgres database.

### Base ##
This class has a constructor that establishes a database connection and returns a reference for database-related CRUD operations.

### Database Configuration Parameter ###
A .env file (or environment variables) with configuration parameters described below are needed to establish a database connection:
```
# production database credentials
PROD_DB_USER=postgres-database-user
PROD_DB_PASSWORD=postgres-database-password
PROD_DB_HOST=database-host
PROD_DB_PORT=database-port

# test database credentials
TEST_DB_USER=postgres-database-user
TEST_DB_PASSWORD=postgres-database-password
TEST_DB_HOST=database-host
TEST_DB_PORT=database-port
```