Function _get_select_comparisons

examples/sharding/separate_tables.py:198–244

Search a Select or Query object for binary expressions. Returns expressions which match a Column against one or more literal values as a list of tuples of the form (column, operator, values). "values" is a single value or tuple of values depending on the operator.

(statement)

def _get_select_comparisons(statement):
    """Search a Select or Query object for binary expressions.

    Returns expressions which match a Column against one or more
    literal values as a list of tuples of the form
    (column, operator, values).   "values" is a single value
    or tuple of values depending on the operator.

    """
    binds = {}
    clauses = set()
    comparisons = []

    def visit_bindparam(bind):
        # visit a bind parameter.

        value = bind.effective_value
        binds[bind] = value

    def visit_column(column):
        clauses.add(column)

    def visit_binary(binary):
        if binary.left in clauses and binary.right in binds:
            comparisons.append(
                (binary.left, binary.operator, binds[binary.right])
            )

        elif binary.left in binds and binary.right in clauses:
            comparisons.append(
                (binary.right, binary.operator, binds[binary.left])
            )

    # here we will traverse through the query's criterion, searching
    # for SQL constructs.  We will place simple column comparisons
    # into a list.
    if statement.whereclause is not None:
        visitors.traverse(
            statement.whereclause,
            {},
            {
                "bindparam": visit_bindparam,
                "binary": visit_binary,
                "column": visit_column,
            },
        )
    return comparisons


# further configure create_session to use these functions