groupby & merge; we are always performing a left-by type operation

    Parameters
    ----------
    by: field to group
    on: duplicates field
    left: left frame
    right: right frame
    _merge_pieces: function for merging
    check_duplicates: boolean, default True
        should we check & clean duplicates

def _groupby_and_merge(by, on, left, right, _merge_pieces,
                       check_duplicates=True):
    """
    groupby & merge; we are always performing a left-by type operation

    Parameters
    ----------
    by: field to group
    on: duplicates field
    left: left frame
    right: right frame
    _merge_pieces: function for merging
    check_duplicates: boolean, default True
        should we check & clean duplicates
    """

    pieces = []
    if not isinstance(by, (list, tuple)):
        by = [by]

    lby = left.groupby(by, sort=False)

    # if we can groupby the rhs
    # then we can get vastly better perf
    try:

        # we will check & remove duplicates if indicated
        if check_duplicates:
            if on is None:
                on = []
            elif not isinstance(on, (list, tuple)):
                on = [on]

            if right.duplicated(by + on).any():
                right = right.drop_duplicates(by + on, keep='last')
        rby = right.groupby(by, sort=False)
    except KeyError:
        rby = None

    for key, lhs in lby:

        if rby is None:
            rhs = right
        else:
            try:
                rhs = right.take(rby.indices[key])
            except KeyError:
                # key doesn't exist in left
                lcols = lhs.columns.tolist()
                cols = lcols + [r for r in right.columns
                                if r not in set(lcols)]
                merged = lhs.reindex(columns=cols)
                merged.index = range(len(merged))
                pieces.append(merged)
                continue

        merged = _merge_pieces(lhs, rhs)

        # make sure join keys are in the merged
        # TODO, should _merge_pieces do this?
        for k in by:
            try:
                if k in merged:
                    merged[k] = key
            except KeyError:
                pass

        pieces.append(merged)

    # preserve the original order
    # if we have a missing piece this can be reset
    from pandas.core.reshape.concat import concat
    result = concat(pieces, ignore_index=True)
    result = result.reindex(columns=pieces[0].columns, copy=False)
    return result, lby