title: Objection Relation Mapping with transformed values
body: <p>Let's say I have two tables :</p>
  <pre><code>People:
  
   name : place
  ----- : ------
  ben   : park
  phil  : office
  
  Food:
  
  food_place  : vegetable
  ----------- : ---------
  ben@park    : carrots
  phil@office : potatoes
  </code></pre>
  <p>I'd like to create a Relation Mapping, I tried:</p>
  <pre><code>  static get relationMappings() {
      const knex = People.knex();
      return {
        food: {
          relation: Model.BelongsToOneRelation,
          modelClass: `${__dirname}/Food`,
          join: {
            from: knex.raw('CONCAT(people.name, &quot;@&quot;, people.place)'),
            to: 'food.food_place',
          },
        },
      };
    }
  </code></pre>
  <p>That didn't work because objection expected the <code>from</code> property to match column name (<code>Error: People.relationMappings.food: join.from must have format TableName.columnName</code>).</p>
  <p>The idea was to join tables like:</p>
  <p><code>INNER JOIN Food ON Food.food_place = CONCAT(people.name, &quot;@&quot;, people.place)</code></p>
  <p><strong>QUESTION</strong> : Is there a was i can do this with relation mappings, or should i use modifiers instead ?</p>