[D7] Unable to use Condition objects with joins

Problem/Motivation

This code does not work:

    $query = db_select('table_a', 'a');
    $join_condition = db_and();
    foreach ($fields as $field) {
      $field = db_escape_field($field);
      $join_condition->where("a.$field = b.$field");
    }
    $query->innerJoin('table_b', 'b', $join_condition);

The reason is that the condition is not compiled, and thus converting the query to string breaks.

Proposed resolution

Backport the D8 fix committed in #2275519: Unable to use Condition objects with joins.

Remaining tasks

Reviews

User interface changes

None

API changes

None

Data model changes

None