实体框架检查在OnModelCreating期间是否存在列 [英] Entity Framework check if column exists during OnModelCreating

问题描述

我正在维护正在写入另一个应用程序(B)的表的该应用程序(A).问题是A写入许多B，并且模型在B的较新版本中已更改.

I am maintaining this application (A) that is writing to the table of another application (B). The problem is that A writes to many B's and that the models have changed in newer versions of B.

示例:A的实体狗"的名称为:姓名，年龄，性别
在大多数情况下，B与表匹配.但是最新版本的B Dog具有以下列:名称，年龄，性别，FavoritFood(不允许为空)

Example: A has the entity Dog with columns: Name, Age, Sex
In most cases of B, this entity matches the table. But the newest version of B Dog has the columns: Name, Age, Sex, FavoritFood (which does not allow nulls)

我既不能从代码也不能从sql服务器更改B的数据库方案.如果这样做的话，B会根据需要重新设计它.我可以更改A的Dog实体，但这需要在B的新旧版本之间进行区分.

I can not change the database scheme of B, neither from code nor the sql server. If I do so, B would just redesign it to its needs. I can alter the Dog entity of A but this would need a distinction between newer and older versions of B.

A使用Entity Framework 6.2作为ORM.

A uses Entity Framework 6.2 as ORM.

到目前为止，我的想法是:检查列是否存在，如果不忽略该字段.

My idea so far was as follows: Check if column exists, if not ignore the field.

protected override void OnModelCreating(DbModelBuilder builder) {
    base.OnModelCreating(builder);
    if (!Database.CompatibleWithModel(true)) {
        builder.Entity<Dog>().Ignore(_ => _.FavoritFood);
    }
}

我不仅不能从OnModelCreating内部访问上下文，而且我发现这种可能性很低，因为它非常通用，我想专门检查一下FavoritFood列.

Not only can't I access the context from within the OnModelCreating I also I find this possibility lacking, since it is very generic and I would like to check specifically for the FavoritFood column.

我该怎么做?

推荐答案

对于其他偶然发现此问题的人:我最后扩展了@ trashr0x评论

For anyone else who stumbles upon this: I ended up expanding on @trashr0x comment

protected override void OnModelCreating(DbModelBuilder builder) 
{
    base.OnModelCreating(builder);
    var exists = CheckIfColumnOnTableExists("Dog", "FavoritFood");
    if(!exists)
        builder.Entity<Dog>().Ignore(_ => _.FavoritFood);
}


private bool CheckIfColumnOnTableExists(string table, string column) {
    using (var context = new DbContext(this.Database.Connection.ConnectionString)) 
    {
        var result = context.Database.SqlQuery<int>($@"SELECT Count(*)
            FROM INFORMATION_SCHEMA.COLUMNS
            WHERE TABLE_NAME = '{table}'
            AND COLUMN_NAME = '{column}'").Single();
        return result == 1;
    }
}

它似乎可以连续工作，如果有人有其他方法，请告诉我:)

It seems to work consistently, if someone has some other way, let me know :)

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