fixes issue #5808 removeColumn tries to delete nonexistent foreign keys (#5851)

changelog.md

 # Future
 - [FIXED] Postgres DECIMAL precision. (PostgreSQL) [#4893](https://github.com/sequelize/sequelize/issues/4893)
+- [FIXED] removeColumn tries to delete non-existant foreign key constraint (mysql) [#5808](https://github.com/sequelize/sequelize/issues/5808)
 
 # 3.23.0
 - [FIXED] Invalid query generated when using LIKE + ANY [#5736](https://github.com/sequelize/sequelize/issues/5736)

lib/dialects/mysql/query-generator.js

@@ -357,8 +357,11 @@ var QueryGenerator = {
     return [
       'SELECT CONSTRAINT_NAME as constraint_name',
       'FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE',
-      'WHERE TABLE_NAME = ' + wrapSingleQuote(tableName),
+      'WHERE (REFERENCED_TABLE_NAME = ' + wrapSingleQuote(tableName),
+        'AND REFERENCED_COLUMN_NAME = ' + wrapSingleQuote(columnName),
+        ') OR (TABLE_NAME = ' + wrapSingleQuote(tableName),
         'AND COLUMN_NAME = ' + wrapSingleQuote(columnName),
+        ')',
     ].join(' ');
   },
 

test/integration/query-interface.test.js

@@ -696,7 +696,7 @@ describe(Support.getTestDialectTeaser('QueryInterface'), function() {
 
     it('should get a list of foreign keys for the table', function() {
       var sql = this.queryInterface.QueryGenerator.getForeignKeysQuery('hosts', this.sequelize.config.database);
-
+      var self = this;
       return this.sequelize.query(sql, {type: this.sequelize.QueryTypes.FOREIGNKEYS}).then(function(fks) {
         expect(fks).to.have.length(3);
         var keys = Object.keys(fks[0]),
@@ -714,6 +714,18 @@ describe(Support.getTestDialectTeaser('QueryInterface'), function() {
         } else {
           console.log('This test doesn\'t support ' + dialect);
         }
+        return fks;
+      }).then(function(fks){
+        if (dialect === 'mysql') {
+          return self.sequelize.query(
+              self.queryInterface.QueryGenerator.getForeignKeyQuery('hosts', 'admin'),
+              {}
+            )
+            .spread(function(fk){
+              expect(fks[0]).to.deep.eql(fk[0]);
+            });
+        }
+        return;
       });
     });
   });