BUG: can't mutate and then drop

[tsdlovell]

On current master

git log | head -n 3
commit 5cafaee7369403a261b3e5df2ddb0c9968aa85f9
Author: Phillip Cloud <cpcloud@gmail.com>
Date:   Wed Jan 24 08:44:17 2018 -0500

I can't do table.mutate(b=table.a).drop(['a'])

In [1]: import ibis

In [2]: import pandas as pd

In [3]: client = ibis.pandas.connect({
   ...:         'df': pd.DataFrame({'a': [1, 2, 3]}),
   ...:         'df_unknown': pd.DataFrame({
   ...:             'array_of_strings': [['a', 'b'], [], ['c']],
   ...:         }),
   ...:     })
   ...:     

In [4]: table = client.table('df')

In [5]: table.mutate(b=table.a).drop(['a'])
---------------------------------------------------------------------------
IbisTypeError                             Traceback (most recent call last)
<ipython-input-5-2f4c8dc18c5a> in <module>()
----> 1 table.mutate(b=table.a).drop(['a'])

/nas/dft/ire/dlovell/gitlab/ibis/ibis/expr/api.py in _table_drop(self, fields)
   2829         raise KeyError('Fields not in table: {0!s}'.format(fields))
   2830 
-> 2831     return self.projection(to_project)
   2832 
   2833 

/nas/dft/ire/dlovell/gitlab/ibis/ibis/expr/api.py in projection(table, exprs)
   2760     projector = L.Projector(table, exprs)
   2761 
-> 2762     op = projector.get_result()
   2763     return TableExpr(op)
   2764 

/nas/dft/ire/dlovell/gitlab/ibis/ibis/expr/analysis.py in get_result(self)
    761 
    762         if len(roots) == 1 and isinstance(roots[0], ops.Selection):
--> 763             fused_op = self._check_fusion(roots[0])
    764             if fused_op is not None:
    765                 return fused_op

/nas/dft/ire/dlovell/gitlab/ibis/ibis/expr/analysis.py in _check_fusion(self, root)
    773         can_fuse = False
    774 
--> 775         resolved = _maybe_resolve_exprs(root.table, self.input_exprs)
    776         if not resolved:
    777             return None

/nas/dft/ire/dlovell/gitlab/ibis/ibis/expr/analysis.py in _maybe_resolve_exprs(table, exprs)
    820 def _maybe_resolve_exprs(table, exprs):
    821     try:
--> 822         return table._resolve(exprs)
    823     except AttributeError:
    824         return None

/nas/dft/ire/dlovell/gitlab/ibis/ibis/expr/types.py in _resolve(self, exprs)
    797         out_exprs = []
    798         for expr in exprs:
--> 799             expr = self._ensure_expr(expr)
    800             out_exprs.append(expr)
    801         return out_exprs

/nas/dft/ire/dlovell/gitlab/ibis/ibis/expr/types.py in _ensure_expr(self, expr)
    803     def _ensure_expr(self, expr):
    804         if isinstance(expr, six.string_types):
--> 805             return self[expr]
    806         elif isinstance(expr, six.integer_types):
    807             return self[self.schema().name_at_position(expr)]

/nas/dft/ire/dlovell/gitlab/ibis/ibis/expr/types.py in __getitem__(self, what)
    728     def __getitem__(self, what):
    729         if isinstance(what, six.string_types + six.integer_types):
--> 730             return self.get_column(what)
    731 
    732         if isinstance(what, slice):

/nas/dft/ire/dlovell/gitlab/ibis/ibis/expr/types.py in get_column(self, name)
    846         column : array expression
    847         """
--> 848         ref = TableColumn(name, self)
    849         return ref.to_expr()
    850 

/nas/dft/ire/dlovell/gitlab/ibis/ibis/expr/types.py in __init__(self, name, table_expr)
    423         if name not in schema:
    424             raise com.IbisTypeError(
--> 425                 "'{0}' is not a field in {1}".format(name, table_expr.columns)
    426             )
    427 

IbisTypeError: 'b' is not a field in ['a']