MySQL: drop foreign key before dropping column

lib/dialects/mysql/query-generator.js

@@ -332,6 +332,26 @@ var QueryGenerator = {
   },
 
   /**
+   * Generates an SQL query that returns the foreign key constraint of a given column.
+   *
+   * @param  {String} tableName  The name of the table.
+   * @param  {String} columnName The name of the column.
+   * @return {String}            The generated sql query.
+   */
+  getForeignKeyQuery: function(table, columnName) {
+    var tableName = table.tableName || table;
+    if (table.schema) {
+        tableName = table.schema + '.' + tableName;
+    }
+    return [
+      'SELECT CONSTRAINT_NAME as constraint_name',
+      'FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE',
+      'WHERE TABLE_NAME = ' + wrapSingleQuote(tableName),
+        'AND COLUMN_NAME = ' + wrapSingleQuote(columnName),
+    ].join(' ');
+  },
+
+  /**
    * Generates an SQL query that removes a foreign key from a table.
    *
    * @param  {String} tableName  The name of the table.
@@ -343,4 +363,9 @@ var QueryGenerator = {
   }
 };
 
+// private methods
+function wrapSingleQuote(identifier){
+  return Utils.addTicks(identifier, '\'');
+}
+
 module.exports = Utils._.extend(Utils._.clone(require('../abstract/query-generator')), QueryGenerator);

lib/dialects/mysql/query-interface.js

+'use strict';
+
+/**
+ Returns an object that treats MySQL's inabilities to do certain queries.
+
+ @class QueryInterface
+ @static
+ */
+
+var _ = require('lodash');
+
+/**
+  A wrapper that fixes MySQL's inability to cleanly remove columns from existing tables if they have a foreign key constraint.
+
+  @method removeColumn
+  @for    QueryInterface
+
+  @param  {String} tableName     The name of the table.
+  @param  {String} columnName    The name of the attribute that we want to remove.
+  @param  {Object} options
+ */
+var removeColumn = function (tableName, columnName, options) {
+  var self = this;
+  options = options || {};
+
+  return self.sequelize.query(
+      self.QueryGenerator.getForeignKeyQuery(tableName, columnName),
+      _.assign({ raw: true }, options)
+    )
+    .spread(function (results) {
+      if (!results.length) {
+        // No foreign key constraints found, so we can remove the column
+        return;
+      }
+      return self.sequelize.query(
+          self.QueryGenerator.dropForeignKeyQuery(tableName, results[0].constraint_name),
+          _.assign({ raw: true }, options)
+        );
+    })
+    .then(function () {
+      return self.sequelize.query(
+          self.QueryGenerator.removeColumnQuery(tableName, columnName),
+          _.assign({ raw: true }, options)
+        );
+    });
+};
+
+module.exports = {
+  removeColumn: removeColumn
+};

lib/query-interface.js

@@ -5,6 +5,7 @@ var Utils = require('./utils')
   , DataTypes = require('./data-types')
   , SQLiteQueryInterface = require('./dialects/sqlite/query-interface')
   , MSSSQLQueryInterface = require('./dialects/mssql/query-interface')
+  , MySQLQueryInterface = require('./dialects/mysql/query-interface')
   , Transaction = require('./transaction')
   , Promise = require('./promise')
   , QueryTypes = require('./query-types');
@@ -360,8 +361,11 @@ QueryInterface.prototype.removeColumn = function(tableName, attributeName, optio
       // sqlite needs some special treatment as it cannot drop a column
       return SQLiteQueryInterface.removeColumn.call(this, tableName, attributeName, options);
     case 'mssql':
-      // mssql needs special treatment as it cannot drop a column with a default constraint
+      // mssql needs special treatment as it cannot drop a column with a default or foreign key constraint
       return MSSSQLQueryInterface.removeColumn.call(this, tableName, attributeName, options);
+    case 'mysql':
+      // mysql needs special treatment as it cannot drop a column with a foreign key constraint
+      return MySQLQueryInterface.removeColumn.call(this, tableName, attributeName, options);
     default:
         var sql = this.QueryGenerator.removeColumnQuery(tableName, attributeName);
         return this.sequelize.query(sql, options);