如何在运行时从 DbContext 获取表名

Question

我试图在运行时从 DbContext 获取数据库表名，我发现 posts 谈到在 EF 4.x 中无法获取 SSpace 项目（以及表名）默认情况下还没有用于这些操作的公共 API。

但我尝试对此进行一些测试，并且我能够在运行时使用调试器从 DbContext 获取表名

此语法由 Visual Studio 生成

((System.Data.Entity.DbContext)(context)).System.Data.Entity.Infrastructure.IObjectContextAdapter.ObjectContext.MetadataWorkspace._itemsSSpace

我做了一些修改，使它在代码中可用，并屈服于这个

var objContext = (context as IObjectContextAdapter).ObjectContext;
var metaData = objContext.MetadataWorkspace;
var items = metaData.GetItems(DataSpace.SSpace);

虽然它遵循相同的目标，但 items 抛出异常 The space 'SSpace' has no associated collection.

• 首先：为什么会出现这个错误。
• 二：有什么办法可以得到这个SSpcae项，或者表名？？

Answer 1

这是运行时的一种方式。

        public static List<string> GetTableNames()
    {
        List<string> tableNameList = new List<string>();
        // use DBContext to get ObjectContext
        DatabaseContext db = new DatabaseContext();
        IObjectContextAdapter adapter = db as IObjectContextAdapter;
        System.Data.Objects.ObjectContext objectContext = adapter.ObjectContext;

        ReadOnlyCollection<EntityType> allTypes = objectContext.MetadataWorkspace.GetItems<EntityType>(DataSpace.CSpace);
        foreach (EntityType item in allTypes)
        {
            // Create full assembly name
            string typeName = "original.poco.namespace." + item.Name;
            Type type = Type.GetType(typeName);

            // wrap into a function
            string sql = db.Set(type).ToString();
            Regex regex = new Regex(@"FROM \[dbo\]\.\[(?<table>.*)\] AS");
            Match match = regex.Match(sql);
            tableNameList.Add( match.Groups["table"].Value);
        }

        return tableNameList;
    }

Answer 2

您会收到该错误，因为在您执行某些操作（执行查询）以使 EF 需要存储集合之前，不会填充存储集合。但即使你拥有它，它对你也没有好处。您可以通过这种方式获取表名列表，但到实体的映射位于 CSSpace 项目集合中，这是完全不可访问的，因为它使用 EF 运行时内部的类型。如果您有实体 A 和 B 以及表 C 和 D，即使您知道您有直接的一对一映射，您也无法确定 A 是映射到 C 还是映射到 D。

Answer 3

这是今天为 Microsoft 的 Northwind 示例数据库开发的一些代码，演示了如何从实体数据模型中获取表列表，然后显示在 DataGridView 中选择的表中的前 x 行。

按原样运行以下代码：

1. 创建一个 Windows 窗体应用程序
2. 为 Northwind DB 添加实体数据模型，将其命名为“NorthwindModel”
3. 添加两个组合框和一个 datagridview（tablesComboBox、topXComboBox 和 dataGridViewForOutput）
4. 将项目添加到 topXComboBox（全部、前 5、前 10 等（必须是 5 的倍数才能使代码工作）
5. 将表单名称更改为 mainForm
6. 突出显示表单文件中的所有代码并替换为以下内容

    using System;
    using System.Data.Entity;
    using System.Linq;
    using System.Reflection;
    using System.Windows.Forms;

    namespace EntityFrameworkBrowser
    {
        public partial class mainForm : Form
        {

            public mainForm()
            {
                InitializeComponent();
            }

            private void mainForm_Load(object sender, EventArgs e)
            {
                Type type = typeof(NorthwindModelEntities);
                var query = type.GetProperties().Where(p => p.GetMethod.ToString().ToLower().Contains(".dbset")).Select(m => m.Name);
                tablesComboBox.DataSource = query.ToList();
                topXComboBox.SelectedIndex = 1;
            }

            private async void BindTableData()
            {
                // Ensure the form has been initialised
                if (topXComboBox.SelectedIndex.Equals(-1))
                {
                    return;
                }
                // Get the DB context 
                NorthwindModelEntities dbContext = new NorthwindModelEntities();
                // Get a reference to the type of the model
                Type type = typeof(NorthwindModelEntities);
                // Get the table name selected by the user in the combo box
                string tableName = tablesComboBox.SelectedItem.ToString();
                // Get a reference to the DbSet from the model
                var prop = type.GetProperty(tableName);
                // Get a reference to the getter for the DbSet
                MethodInfo get = prop.GetMethod;
                // Invoke the getter for the DbSet 
                object tableContent = get.Invoke(dbContext, null);
                // Create a query that will return all records from the selected table
                IQueryable query = (IQueryable)tableContent;
                // Find out how many records the user has requested, All, Top5, Top 10, etc
                int count = topXComboBox.SelectedIndex * 5;
                // If a value other than all (selected index 0) has been selected the query needs to be refactored
                if (count != 0)
                {
                    // Get the element type for the DbSet from the entity data model
                    Type returnType = query.ElementType;
                    // Get a reference to the 'Take' extension method 
                    MethodInfo takeMethod = (MethodInfo)typeof(Queryable).GetMethod("Take");
                    // Make a generic version of the 'Take' method
                    MethodInfo m = takeMethod.MakeGenericMethod(returnType);
                    // Refactor the query to take the top X records based on the user combo box selection
                    query = (IQueryable)m.Invoke(null, new object[] { query, count });
                }
                // Execute the query and bind the results to the data grid view
                dataGridViewForOutput.DataSource = await query.ToListAsync();
            }

            private void tablesComboBox_SelectedIndexChanged(object sender, EventArgs e)
            {
                BindTableData();
            }

            private void topXComboBox_SelectedIndexChanged(object sender, EventArgs e)
            {
                BindTableData();
            }
        }
    }

希望 cmets 应该解释发生了什么，你不会猜到的。

希望它对某人有所帮助，因为它需要一些锻炼

:0)

Answer 4

我到处都在使用这些类；应该证明是有帮助的。与 EF4 或 5 一起使用。要让 ObjectContext 从 DbContext 传递到顶级 EntityNavigationList.List，请使用 ((IObjectContextAdapter)myDbContext).ObjectContext。表名的简单列表将是 EntityNavigationList.List 中每个项目的 FromEntity。

public class NavigationItem
{
    public string FromEntity { get; set; }
    public string ToEntity { get; set; }
    public RelationshipMultiplicity FromMultiplicity { get; set; }
    public RelationshipMultiplicity ToMultiplicity { get; set; }
    public string ForeignKeyColumn { get; set; }
    public string PrimaryKeyColumn { get; set; }
    public EntitySetBase EntitySet { get; set; }

    /// <summary>
    /// Entity type may be "Formula"; entity set may be "Formulae"
    /// </summary>
    public string EntitySetName { get { return EntitySet == null ? string.Empty : EntitySet.Name; } }
}  

public static class EntityNavigationList
{
    private static List<NavigationItem> _navItems;

    public static List<NavigationItem> List( ObjectContext context )
    {
        if ( _navItems == null )
        {
            InitializeNavigationItems( context );
        }
        return _navItems;
    }

    /// <summary>
    /// Create a list of all navigation items in the model
    /// </summary>
    private static void InitializeNavigationItems( ObjectContext context )
    {
        var entityMetadata = context.MetadataWorkspace.GetItems( DataSpace.CSpace );
        var entitySetMetadata = context.MetadataWorkspace.GetEntityContainer( context.DefaultContainerName, DataSpace.CSpace ).BaseEntitySets;

        var query = from meta in entityMetadata
                      .Where( m => m.BuiltInTypeKind == BuiltInTypeKind.EntityType )
                    from p in ( meta as EntityType ).NavigationProperties
                    let foreignKey = ( p.ToEndMember.DeclaringType as AssociationType ).ReferentialConstraints.FirstOrDefault()
                    let primaryKey = ( meta as EntityType ).KeyMembers
                    select new NavigationItem
                    {
                        FromEntity = p.FromEndMember.Name,
                        ToEntity = p.ToEndMember.Name,
                        FromMultiplicity = p.FromEndMember.RelationshipMultiplicity,
                        ToMultiplicity = p.ToEndMember.RelationshipMultiplicity,
                        ForeignKeyColumn = foreignKey == null ? string.Empty : foreignKey.ToProperties.First().ToString(),
                        PrimaryKeyColumn = primaryKey == null ? string.Empty : primaryKey.First().Name,
                        /// We need the corresponding entity set so we can get it's name (formula belongs to entity set "formulae")
                        EntitySet = ( from moMeta in entitySetMetadata
                                      where moMeta.ElementType.Name == p.FromEndMember.Name
                                      select moMeta ).FirstOrDefault()
                    };

        _navItems = query.ToList();
    }

}