Update docs for unique

[kmalakoff]

$unique docs

Common

• $unique with $sort
  • Rows will contain values for the first entry matching the $unique field sorted by the $sort query.
• $unique with $count
  • All other queries are ignored and the count of rows after grouping by the $unique fields is returned.

    backbone-sql

$unique uses postgres specific functions for some functionality.

• $unique without specifying $select or $values:
  • The documents returned will contain fields for each column. This uses postgres window functions and will not work with mysql.
• $unique with $select or $values when $select or $values contain fields not in $unique.
  • The documents returned will contain fields for each field in ($select or $values). This uses postgres window functions and will not work with mysql.
• $unique with $select or $values when $select or $values do not contain fields not in $unique.
  • As above, but an sql distinct query is used. This will work with mysql, etc.
• $unique without $sort
  • Rows will contain values for the first entry matching the $unique field. The 'first entry' is the first when rows are sorted by id.

    backbone-mongo

$unique uses the mongodb aggregation framework. There are limitations involved, so results are a bit different than with postgres.

• $unique without specifying $select or $values:
  • This will only return the id field and the fields specified in $unique.
• $unique with $select or $values:
  • The document returned with have each field in $select or $values.
• $unique without $sort
  • Rows will contain values for the first entry matching the $unique field. The 'first entry' is random without a $sort query. Don't do this.