C# 实体框架代码优先 - 忽略数据库中缺失的列

Question

I have a project that connects to many different SQL Server databases, all with different versions of the schema. That means that some columns is only in some of the databases.

I am using Entity Framework with a code-first approach, and my entity models are always up-to-date, so what I want is that when I select something from the database, I want the missing columns to be null when mapped to the model.

In this example, I want "Col2" to be null, because "Col2_Missing" does not exist in the database:

var result = dbContext.Set<DemoData>().Select(x => new
{
    Col1 = x.Col1,
    Col2 = x.Col2_Missing
}.ToList();

However, the code throws an exception:

System.NotSupportedException: 'The specified type member 'Col2_Missing' is not supported in LINQ to Entities. Only initializers, entity members, and entity navigation properties are supported.'

I have tried making the property .IsOptional() , but as I understand that just makes the property nullable when the column is created.

The current solution is checking the schema version before the select, and then have a lot of ugly "if" in the code.

Answer 1

You might want to try dapper. It's been a couple minutes since I used it, but it seems like dapper would automagically ignore attributes in the model that are not present in the resultset and also ignore attributes in the resultset that are not present in the model.

However, the usage is different. You wouldn't be using Linq to query. Instead, you would do something similar to an ADO.Net SQL Command. Execute a string sql statement or a string that represents a stored procedure in the database.

Dapper is a big step above SQL Commands though since it automaps the results of the sql to collection of typed objects. You don't need to loop through a SQL datareader. You can also compose sql statements with curly brace variables... and dapper will automatically parameterize the statement for you... which helps prevent SQL injection.