Sorting broken after clicking different column headers

[jberes]

Description

Sorting stops working after sorting several columns

• igniteui-angular version: 6.1
• browser: Chrome

Steps to reproduce

1. Step 1
2. Step 2
3. ...

Result

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Expected result

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[kdinev]

@jberes You have applied sorting on the Order Date column, which has unique dates, and then your applying a second sort on the Units in Stock column, which doesn't apply any additional sorting, since there's no records that repeat the same date. The sorting by the second column is applied to records that have identical values for the first sorting.
It's the same with groupby: when you group by a column with unique values, and sort by a different column, nothing changes, since there's never more than 1 record in the groups to resort by.