Added support for expressions in keypath building

[papafe]

Fixes #3494

TODO

• Changelog entry
• Tests

[coveralls-official]

Pull Request Test Coverage Report for Build 8160379640

Details

• 13 of 14 (92.86%) changed or added relevant lines in 1 file are covered.
• 13 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+3.0%) to 80.915%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
Realm/Realm/DatabaseTypes/KeyPathCollection.cs	13	14	92.86%

Files with Coverage Reduction	New Missed Lines	%
Realm/Realm/Native/HttpClientTransport.cs	13	50.0%

Totals
Change from base Build 8157624912:	3.0%
Covered Lines:	6676
Relevant Lines:	8107

---

💛 - Coveralls

[nirinchev]

Looks good - one thing to consider is if we want to do anything about traversing collections. E.g. this expression could enable calls like First, Single, etc.:

MethodCallExpression
{ 
    Arguments:
    [
        MemberExpression
        {
            Expression: not null,
            Member: PropertyInfo argumentPi 
        } innerExpression
    ]
} when typeof(IEnumerable).IsAssignableFrom(argumentPi.PropertyType)
    => GetFullPath(innerExpression)

This would enable stuff like:

// Evaluates to "Friends.FirstName"
KeyPath.For<Person>(p => p.Friends.First().FirstName);

Might be a bit magical though and obviously doesn't include much validation.

[nirinchev]

Is the For name something from Swift/ObjC? I guess I understand where it's coming from, but it's also kind of unusual for a .NET API name.

[papafe]

No, it's something I thought made sense for this case 😁
I also thought it was following a similar "style" of KeyPathCollection.Of. Do you have any other name suggestion? I would much more prefer to have a static method than using the constructor to be honest.

[papafe]

@nirinchev, do you have any suggestion here about an alternative name?

[nirinchev]

I think it'd be impossible to use the ctor here, because we need the generic arg. Perhaps it's a bit verbose, but how about FromExpression?

[papafe]

I think it sounds reasonable :)

[papafe]

Looks good - one thing to consider is if we want to do anything about traversing collections. E.g. this expression could enable calls like First, Single, etc.:

MethodCallExpression
{ 
    Arguments:
    [
        MemberExpression
        {
            Expression: not null,
            Member: PropertyInfo argumentPi 
        } innerExpression
    ]
} when typeof(IEnumerable).IsAssignableFrom(argumentPi.PropertyType)
    => GetFullPath(innerExpression)

This would enable stuff like:

// Evaluates to "Friends.FirstName"
KeyPath.For<Person>(p => p.Friends.First().FirstName);

Might be a bit magical though and obviously doesn't include much validation.

Yeah, it would work, but I think it would feel forced in a way, and could give the wrong impression that we're creating an index path for a specific collection index. For now I'd prefer to keep it as it is, and we can come back to it if there's request for more strongly typed keypaths.