System.Data.SQLite
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//---------------------------------------------------------------------
// <copyright file="SymbolPair.cs" company="Microsoft">
//      Portions of this file copyright (c) Microsoft Corporation
//      and are released under the Microsoft Pulic License.  See
//      http://archive.msdn.microsoft.com/EFSampleProvider/Project/License.aspx
//      or License.txt for details.
//      All rights reserved.
// </copyright>
//---------------------------------------------------------------------

namespace System.Data.SQLite
{
  using System;
  using System.Collections.Generic;
  using System.Diagnostics;
  using System.IO;
  using System.Text;
  using System.Data.Metadata.Edm;
  using System.Data.Common.CommandTrees;

  /// <summary>
  /// The SymbolPair exists to solve the record flattening problem.
  /// <see cref="SqlGenerator.Visit(DbPropertyExpression)"/>
  /// Consider a property expression D(v, "j3.j2.j1.a.x")
  /// where v is a VarRef, j1, j2, j3 are joins, a is an extent and x is a columns.
  /// This has to be translated eventually into {j'}.{x'}
  /// 
  /// The source field represents the outermost SqlStatement representing a join
  /// expression (say j2) - this is always a Join symbol.
  /// 
  /// The column field keeps moving from one join symbol to the next, until it
  /// stops at a non-join symbol.
  /// 
  /// This is returned by <see cref="SqlGenerator.Visit(DbPropertyExpression)"/>,
  /// but never makes it into a SqlBuilder.
  /// </summary>
  class SymbolPair : ISqlFragment
  {
    public Symbol Source;
    public Symbol Column;

    public SymbolPair(Symbol source, Symbol column)
    {
      this.Source = source;
      this.Column = column;
    }

    #region ISqlFragment Members

    public void WriteSql(SqlWriter writer, SqlGenerator sqlGenerator)
    {
      // Symbol pair should never be part of a SqlBuilder.
      Debug.Assert(false);
    }

    #endregion
  }
}