LINQ to MySQL, based on Matt Warren's provider

On each release Matt Warren’s IQueryable provider is getting more and more interesting. On this post I’ll show you how easy it is to plug-in your custom QueryFormatter to start querying MySQL databases.

Step 1: Download Matt Warren’s provider

The code on this post is based on release 11. Although I cannot ensure it will work with newer releases it is always recommended that you download the latest.

Step 2: Create a custom QueryFormatter class

The following code is a custom QueryFormatter for MySQL.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text;

namespace Sample {

   internal class MySqlQueryFormatter : QueryFormatter {

      protected override Expression VisitNamedValue(NamedValueExpression value) {
         sb.Append("?" + value.Name);
         return value;
      }

      protected override Expression VisitSelect(SelectExpression select) {
         sb.Append("SELECT ");
         if (select.IsDistinct) {
            sb.Append("DISTINCT ");
         }

         if (select.Columns.Count > 0) {
            for (int i = 0, n = select.Columns.Count; i < n; i++) {
               ColumnDeclaration column = select.Columns[i];
               if (i > 0) {
                  sb.Append(", ");
               }
               ColumnExpression c = this.VisitValue(column.Expression) as ColumnExpression;
               if (!string.IsNullOrEmpty(column.Name) && (c == null || c.Name != column.Name)) {
                  sb.Append(" AS ");
                  sb.Append(column.Name);
               }
            }
         } else {
            sb.Append("NULL ");
            if (this.isNested) {
               sb.Append("AS tmp ");
            }
         }
         if (select.From != null) {
            this.AppendNewLine(Indentation.Same);
            sb.Append("FROM ");
            this.VisitSource(select.From);
         }
         if (select.Where != null) {
            this.AppendNewLine(Indentation.Same);
            sb.Append("WHERE ");
            this.VisitPredicate(select.Where);
         }
         if (select.GroupBy != null && select.GroupBy.Count > 0) {
            this.AppendNewLine(Indentation.Same);
            sb.Append("GROUP BY ");
            for (int i = 0, n = select.GroupBy.Count; i < n; i++) {
               if (i > 0) {
                  sb.Append(", ");
               }
               this.VisitValue(select.GroupBy[i]);
            }
         }
         if (select.OrderBy != null && select.OrderBy.Count > 0) {
            this.AppendNewLine(Indentation.Same);
            sb.Append("ORDER BY ");
            for (int i = 0, n = select.OrderBy.Count; i < n; i++) {
               OrderExpression exp = select.OrderBy[i];
               if (i > 0) {
                  sb.Append(", ");
               }
               this.VisitValue(exp.Expression);
               if (exp.OrderType != OrderType.Ascending) {
                  sb.Append(" DESC");
               }
            }
         }
         if (select.Take != null) {
            this.AppendNewLine(Indentation.Same);
            sb.Append("LIMIT ");
            this.Visit(select.Take);
            sb.Append(" ");
         }
         if (select.Skip != null) {
            sb.Append("OFFSET ");
            this.Visit(select.Skip);
            sb.Append(" ");
         }
         return select;
      }
   }
}

Try to build and you’ll notice that MySqlQueryFormatter is trying to access a lot of QueryFormatter members that are private, so you’ll need to make those protected.

Since all interaction with QueryFormatter is done through a single static method called Format, which creates an instance of the same type, we need to change the type of the created instance to MySqlQueryFormatter.

// line 22, QueryFormatter.cs
QueryFormatter formatter = new MySqlQueryFormatter();

This is the fast way to get things working, a better approach would be to have a QueryFormatter property on DbQueryProvider.

Step 3: Comment-out the following two lines in the DbQueryProvider.Translate method

// line 123-124, DbQueryProvider.cs
//expression = SkipRewriter.Rewrite(expression);
//expression = OrderByRewriter.Rewrite(expression);

Since these calls are used to support SQL Server’s complicated syntax for paging, for MySQL we don’t need them.

And that’s it!