How to create or alter a DB schema dynamically (at run time) using Gramex FormHandler

Question

I want to be able to (at run time) create or alter a DB schema dynamically on a particular event (e.g. click of a button) using FormHandler microservice of Gramex.

Answer 1

FormHandler supports defining columns in the spec.

For example, this configuration creates a table called profile with 4 columns: user, password, age and id.

url:
    handler: FormHandler
    kwargs:
      url: 'postgresql://$USER:$PASS@server/db'       # Pick any database
      table: profile              # Pick any table name to create
      id: id                      # The "id" column is primary key
      # Define your table's columns
      columns:
        user: TEXT                # Use any SQL type allowed by DB
        password: VARCHAR(40)     # including customizations
        age:
          type: INTEGER           # You can also specify as a dict
          nullable: true          # Allows NULL values for this field
          default: 0              # that default to zero
        id:
          type: INTEGER           # Define an integer ID column
          primary_key: true       # as a primary key
          autoincrement: true     # that auto-increments

But if this needs to be changed at runtime, e.g. when a user clicks on a button, you can use a FunctionHandler with gramex.data.alter()

For example, add this to your gramex.yaml:

url:
  alter:
    pattern: /alter
    handler: FunctionHandler
    kwargs:
      # You can decide the columns dynamically here
      function: >
        gramex.data.alter(url, table, columns={
           col: 'TEXT' for col in handler.args.get('col', [])
        })

When /alter?col=email is called, the function adds an email column as text.

NOTE: There's no option to DELETE columns.

Answer 2

You can do it using queryfunction of FormHandler which can modify the query based on the query parameters passed in the url.

Refer the link below for more https://gramener.com/gramex/guide/formhandler/#formhandler-queryfunction

Answer 3

You should try this:

In your yaml handler:

queryfunction: mymodule.sales_query(args)

In your python code:

def sales_query(args):
    cities = args.get('ct', [])
    if len(cities) > 0:
        vals = ', '.join("'%s'" % pymysql.escape_string(v) for v in cities)
        return 'SELECT * FROM sales WHERE city IN (%s)' % vals
    else:
        return 'SELECT * FROM sales'

Reference from: https://gramener.com/gramex/guide/formhandler/#formhandler-queryfunction