migration.js

function updateTile(tile) {
  if (tile.get('width')) {
    tile.set('columnCount', (tile.get('width') + gutter) / columnToColumn);
  }
  tile.set('width', tile.get('columnCount') * columnToColumn - gutter);
  return tile.save();
}

function updateTiles() {
  // this is called
  return tileModel.fetchAll().then(function(tiles) {
    // this is never called
    return tiles.mapThen(updateTile);
  });
}

exports.up = function(knex) {
  return knex.schema.table('tile', function (table) {
    table.integer('width');
  }).then(updateTiles).then(function() {
    return knex.schema.table('tile', function (table) {
      table.dropColumn('column_count');
    });
  });
};

exports.down = function(knex) {
  return knex.schema.table('tile', function (table) {
    table.integer('column_count');
  }).then(updateTiles).then(function() {
    return knex.schema.table('tile', function (table) {
      table.dropColumn('width');
    });
  });
};