[MSSQL] Format Merge Statement foreignKey violation as ForeignKeyConstraintError (#7024)

* [MSSQL] MERGE statement foreignKey violation is formatted as ForeignKeyConstraintError, Closes #7011

* Added changelog entry

* Alter changelog entry

* [ci skip] link to issue

changelog.md

 # Future
+- [FIXED] Throw MSSQL MERGE Statement foreignKey violations as ForeignKeyConstraintError [#7011](https://github.com/sequelize/sequelize/pull/7011)
 - [FIXED] Transaction Name too long, transaction savepoints for SQL Server [#6972](https://github.com/sequelize/sequelize/pull/6972)
 - [FIXED] Issue with sync hooks (before/afterInit, before/afterDefine) [#6680](https://github.com/sequelize/sequelize/issues/6680)
 - [FIXED] MSSQL handle large bulk inserts [#6866](https://github.com/sequelize/sequelize/issues/6866)

lib/dialects/mssql/query.js

@@ -246,6 +246,7 @@ class Query extends AbstractQuery {
     match = err.message.match(/Failed on step '(.*)'.Could not create constraint. See previous errors./) ||
             err.message.match(/The DELETE statement conflicted with the REFERENCE constraint "(.*)". The conflict occurred in database "(.*)", table "(.*)", column '(.*)'./) ||
             err.message.match(/The INSERT statement conflicted with the FOREIGN KEY constraint "(.*)". The conflict occurred in database "(.*)", table "(.*)", column '(.*)'./) ||
+            err.message.match(/The MERGE statement conflicted with the FOREIGN KEY constraint "(.*)". The conflict occurred in database "(.*)", table "(.*)", column '(.*)'./) ||
             err.message.match(/The UPDATE statement conflicted with the FOREIGN KEY constraint "(.*)". The conflict occurred in database "(.*)", table "(.*)", column '(.*)'./);
     if (match && match.length > 0) {
       return new sequelizeErrors.ForeignKeyConstraintError({

test/integration/model/upsert.test.js

@@ -389,6 +389,30 @@ describe(Support.getTestDialectTeaser('Model'), function() {
             });
         });
       });
+      
+      if (dialect === 'mssql') {
+        it('Should throw foreignKey violation for MERGE statement as ForeignKeyConstraintError', function () {
+          const User = this.sequelize.define('User', {
+            username: {
+              type: DataTypes.STRING,
+              primaryKey: true
+            }
+          });
+          const Posts = this.sequelize.define('Posts', {
+            title: {
+              type: DataTypes.STRING,
+              primaryKey: true
+            },
+            username: DataTypes.STRING
+          });
+          Posts.belongsTo(User, { foreignKey: 'username' });
+          return this.sequelize.sync({ force: true })
+            .then(() => User.create({ username: 'user1' }))
+            .then(() => {
+              return expect(Posts.upsert({ title: 'Title', username: 'user2' })).to.eventually.be.rejectedWith(Sequelize.ForeignKeyConstraintError);
+            });
+        });
+      }
     });
   }
 });