title: Left join of columns with different datatypes in Slick
body: <p>I have an situation where one column is <code>Bigint</code> in one table and another one is <code>varchar</code>. I am using <code>Scala</code> and <code>Slick</code> for the application. The problem is I need to do a Left Join on these two columns, but getting compilation errors due to the type mismatch. 
  I can change the DB column from BigInt to varchar, but then I will have to make changes in many places. Is there a way in which I can avoid changing the tables and the related entities ?</p>