OrmLite query to select some of the columns from each of 2 joined tables

问题

Following on from this comment, how can I do a ServiceStack OrmLite query that joins two or more tables and returns some of the columns from each of them?

Using the OrmLite Does_only_populate_Select_fields_wildcard unit test as example, I'd like to do something like this:

public class DeptEmployee
{
    [PrimaryKey]
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    [References(typeof(Department2))]
    public int DepartmentId { get; set; }

    [Reference]
    public Department2 Department { get; set; }
}

public class Department2
{
    [PrimaryKey]
    public int Id { get; set; }
    public string Name { get; set; }
}

var q = db.From<DeptEmployee>()
    .Join<Department2>()
    .Select<DeptEmployee, Department2>((de, d2) => new[] { de.FirstName, de.LastName, d2.Name });
var results = db.Select(q);

The above does not return a list of anonymous types containing FirstName, LastName and Name, as I'd expect. It still returns a list of DeptEmployee objects (but with only FirstName and LastName populated).

回答1:

An important thing to note in OrmLite is how the query is constructed and executed is independent to how the results are mapped. It doesn't matter whether the query is raw custom SQL or a Typed SQL Expression, OrmLite only looks at the dataset returned to workout how the results should be mapped.

So when use the Select<T>(SqlExpression<T>) API, OrmLite will always try to map the results into the primary SqlExpression Type in db.From<DeptEmployee>() which isn't what you want since the custom columns you've selected don't match the shape of DeptEmployee POCO.

There are a few different ways to read a custom schema which all work off the same query (as it's independent to how you chose to map the results):

var q = db.From<DeptEmployee>()
    .Join<Department2>()
    .Select<DeptEmployee, Department2>(
        (de, d2) => new { de.FirstName, de.LastName, d2.Name });

Our recommendation, esp. for a typed code-first ORM like OrmLite is to create a Typed Custom POCO and select that, e.g:

class Custom
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Name { get; set; }
}

db.Select<Custom>(q).PrintDump();

Which will print out a nice:

[
    {
        FirstName: First 1,
        LastName: Last 1,
        Name: Dept 1
    },
]

The primary benefit is that you get Typed access to your custom results in a List<Custom>.

If you don't want to create a custom Type you can Select OrmLite's Dynamic Result APIs, e.g:

If you're happy knowing the positions of the different fields you can select a List<object> which will return the selected fields in the order they were selected, e.g:

db.Select<List<object>>(q).PrintDump();

Prints:

[
    [
        First 1,
        Last 1,
        Dept 1
    ],
]

Otherwise if you also want the names returned you can select a string object dictionary, e.g:

db.Select<Dictionary<string,object>>(q).PrintDump();

Which prints results similar to the Custom POCO, but the names and corresponding values are maintained in a loose-typed object Dictionary:

[
    {
        FirstName: First 1,
        LastName: Last 1,
        Name: Dept 1
    },
]

If you were instead only selecting 2 columns, e.g:

var q = db.From<DeptEmployee>()
    .Join<Department2>()
    .Select<DeptEmployee, Department2>(
        (de, d2) => new { de.LastName, d2.Name });

You can make of OrmLite's convenient data access APIs which will let you select 2 columns into a Dictionary<string,string>, e.g:

db.Dictionary<string,string>(q).PrintDump();

Which prints:

{
    Last 1: Dept 1,
    Last 2: Dept 2,
    Last 3: Dept 3
}

Notice this is very different to the string object dictionary above as it returns results in a single Dictionary<string,string> for all rows instead of List<Dictionary<string,object>>, which has a Dictionary for each row.

Likewise if you were only selecting 1 field, e.g:

var q = db.From<DeptEmployee>()
    .Join<Department2>()
    .Select(x => x.LastName);

Then you can select a singular column of results in a List<string>, e.g:

db.Column<string>(q).PrintDump();

Which prints:

[
    Last 1,
    Last 2,
    Last 3
]

If you instead wanted distinct results you can return them in a HashSet<string> with:

db.ColumnDistinct<string>(q).PrintDump();

To return to the original important point, it doesn't matter how the query was constructed (which just controls the SQL that's generated), OrmLite only looks at the returned resultset to Map the results, which it tries to map to the target API that you've specified you want the results mapped into, so executing custom SQL:

db.Column<string>("SELECT LastName FROM DeptEmployee").PrintDump();

Or if you executed a Stored Procedure:

db.Column<string>("EXEC GetLastNamesFromDeptEmployees").PrintDump();

Is mapped exactly the same way if you used a typed SQL Expression, i.e. OrmLite only looks at the resultset which it maps to how you want the results returned.

来源：https://stackoverflow.com/questions/37442401/ormlite-query-to-select-some-of-the-columns-from-each-of-2-joined-tables

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