实体框架：从具有未知类型列的通用表中读取数据

Question

I am new to Entity Framework, and in my project I have to perform a generic query to retrieve the maximum value of a db column. Let's say that this is my main method:

long GetMaxValue(string tableName, string columnName)
{
    // code here
}

and the SQL equivalent is literally

SELECT MAX(<columnName>) FROM <tableName>

I have created a GenericDbEntity class which contains only a NumericColumn property representing this column, and in the implementation of OnModelCreating inside my DbContext I have this:

modelBuilder.Entity<GenericDbEntity>().ToTable(tableName).HasNoKey();
modelBuilder.Entity<GenericDbEntity>().Property(e => e.NumericColumn)
                                      .HasColumnName(columnName)
                                      .HasConversion(p => (?)p, (? p) => Convert.ToInt64(p));

The question marks inside HasConversion are because I don't actually know which is the db type of that column (I only know it's numeric), but I want to convert it to long , since that's the data type of GenericDbEntity.NumericColumn . I tried to replace the question marks with object , but it says:

The property 'GenericDbEntity.NumericColumn' is of type 'object' which is not supported by the current database provider. Either change the property CLR type, or ignore the property using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.

Is there an easy way to achieve this?

PS: I tried also to use dynamic , both in HasConversion and as data type of NumericColumn , but it didn't help.

Edit: The called compiled query is this one:

private static readonly Func<DatabaseContext, long> getMaxValue =
    EF.CompileQuery((DatabaseContext context) =>
                        context.GenericEntity
                               .Max(e => e.NumericColumn));

Answer 1

Added solution which generates LINQ query for particular table and column. For making it working you have to REMOVE HasConversion because properties with converters cannot be used in LINQ queries.

For example for call

var maxValue = context.GetMaxValue("Persons", "id");

Will be generated and executed the following query:

var maxValue = (long)context.Set<Person>().Max(e => e.Id);

And implementation

public static class QueryableExtensions
{
    public static long GetMaxValue(this DbContext ctx, string tableName, string columnName)
    {
        var model = ctx.Model;

        var entityType = model.GetEntityTypes().FirstOrDefault(et => tableName.Equals(et.GetTableName(), StringComparison.InvariantCultureIgnoreCase));

        if (entityType == null)
            throw new InvalidOperationException($"Entity for table '{tableName}' not found.");

        // GetColumnName() can be obsolete, it depends on EF Core version.
        var prop = entityType.GetProperties().FirstOrDefault(p => columnName.Equals(p.GetColumnName(), StringComparison.InvariantCultureIgnoreCase));

        if (prop == null)
            throw new InvalidOperationException($"Property for column '{tableName}'.'{columnName}' not found.");

        var entityParam = Expression.Parameter(entityType.ClrType, "e");
        var ctxParam = Expression.Parameter(typeof(DbContext), "ctx");

        // ctx.Set<entityType>()
        var setQuery = Expression.Call(ctxParam, nameof(DbContext.Set), new[] {entityType.ClrType});

        Expression propExpression;
        if (prop.PropertyInfo == null)
        {
            // 'prop' is Shadow property, so call via EF.Property(e, "name")
            propExpression = Expression.Call(typeof(EF), nameof(EF.Property), new[] { prop.ClrType },
                entityParam, Expression.Constant(prop.Name));
        }
        else
        {
            propExpression = Expression.MakeMemberAccess(entityParam, prop.PropertyInfo);
        }

        // e => e.Prop
        var propLambda = Expression.Lambda(propExpression, entityParam);

        // ctx.Set<entityType>().Max(e => e.Prop)
        Expression maxCall = Expression.Call(typeof(Queryable), nameof(Queryable.Max),
            new[] { entityType.ClrType, prop.ClrType }, 
            setQuery, propLambda);

        // cast if needed
        if (maxCall.Type != typeof(long))
            maxCall = Expression.Convert(maxCall, typeof(long));

        // ctx => ctx.Set<entityType>().Max(e => e.Prop)
        var maxLambda = Expression.Lambda<Func<DbContext, long>>(maxCall, ctxParam);

        // probably we can cache such compiled retrieving function
        var func = maxLambda.Compile();

        return func(ctx);
    }
}