How to Resolve 'Unable to create a constant value' Error in LINQ JOIN with IEnumerable Constant Collection?

LINQ connection using IEnumerableconstant collection

When using a LINQ JOIN clause within a SELECT statement of another JOIN statement, the error "Cannot create a constant value of type API.Models.PersonProtocol. Only primitive or enumerated types are supported in this context" occurs. This error occurs because the IEnumerableCollectionppCombined cannot be used as a constant value in a database context.

In the code provided:

<code class="language-csharp">var persons = db.Favorites
    .Where(x => x.userId == userId)
    .Join(db.Person, x => x.personId, y => y.personId, (x, y) =>
        new PersonDTO
        {
            personId = y.personId,
            addressId = y.addressId,                   
            favoriteId = x.favoriteId,
            personProtocol = (ICollection<PersonProtocol>) ppCombined
                .Where(a => a.personId == x.personId)
                .Select( b => new PersonProtocol()
                 {
                     personProtocolId = b.personProtocolId,
                     activateDt = b.activateDt,
                     personId = b.personId
                 })
        });</code>

This code attempts to concatenate data from the db.Favorites and db.Person tables and then projects the result into a PersonDTO object. Within the PersonDTO object, there is a personProtocol property that is set to the result of a LINQ query against the ppCombined collection.

The problem is trying to use ppCombined as a constant value in a database context. ppCombined is a IEnumerable object constructed in memory, not a database table. When executing a LINQ query, it attempts to convert the personProtocol projection into a SQL statement. However, it cannot do this because ppCombined is not a database table or primitive type.

Solution

To resolve this error, we can fetch the filtered items in ppCombined in memory after retrieving other attributes from the database. The following is the modified code:

<code class="language-csharp">var persons = db.Favorites
    .Where(f => f.userId == userId)
    .Join(db.Person, f => f.personId, p => p.personId, (f, p) =>
        new // 匿名对象
        {
            personId = p.personId,
            addressId = p.addressId,   
            favoriteId = f.favoriteId,
        })
    .AsEnumerable() // 数据库查询在此结束，其余部分是在内存中进行的查询
    .Select(x =>
        new PersonDTO
        {
            personId = x.personId,
            addressId = x.addressId,   
            favoriteId = x.favoriteId,
            personProtocol = ppCombined
                .Where(p => p.personId == x.personId)
                .Select(p => new PersonProtocol
                {
                    personProtocolId = p.personProtocolId,
                    activateDt = p.activateDt,
                    personId = p.personId
                })
                .ToList()
        });</code>

In this modified code, the database query ends with the AsEnumerable() method. This means that the rest of the operation is performed entirely in memory. Then, in a memory-based Select projection of the anonymous object, filter the ppCombined collection and convert it to a list.

By separating the database query from the in-memory processing, we avoid the problem of passing in-memory collections into the database context, thus solving the bug.

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