Add expressions (derived attributes) to the model

[Stiivi]

Add expression for measures and aggregates. Consider using Grako for the compiler.

Interface for the backends

Backends might implement their own arithmetic expression compiler as an object with the following methods:

• compile_literal(literal) – compile a literal object – string, integer. Should return a literal expression object that will be further passed to this compiler.
• compile_operator(operator, left, right) – compile a binary operator. Left/right are expressions generated by this compiler.
• compile_unary(operator, operand) – compile an unary operator. Operand is an expression compiled by this compiler.
• compile_function(func, args) – compile a function with arguments. Args are expressions compiled by this compiler.
• compile_variable(name) – compile identifier/variable/named constant. Name is a string.

The functions should return a compiled expression that might be passed as an argument.

[Stiivi]

Use Expressions

[polin924]

Hi Stefan:
I tried to use the framework 'Expressions' to construct Sqlalchemy object. I followed the example 'sqlalchemy_compiler.py', and changed the following line from

selection = compiler.compile("(amount / transactions) * 2", table)
to
selection = compiler.compile("sum(amount * transactions) / sum(transactions)", table)

When I ran the sqlalchemy_compiler.py, it showed the following error:
TypeError: unsupported operand type(s) for /: 'Function' and 'Function'

How can I solve this issue and construct the Sqlalchemy object I want
thank you

[Stiivi]

@polin924 the example included in the Expressions is not full implementation of SQL alchemy expression compiler, it has just few methods implemented to serve as an example. You need to fully define the compile_function(self, context, function, args) returning a SQLAlchemy function object.