New LINQ support in C#/.NET driver Robert Stam, 10gen

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August 15, 2012
Open source, high performance database
LINQ in the C#/.NET Driver
Robert Stam
Software Engineer, 10gen
Wednesday, August 15, 12

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• Added in the 1.4 release
• Enhanced in 1.4.1, 1.4.2 and 1.5
• Think of LINQ as just another way to write
your queries
• A higher level of abstraction: write your
queries in C#
• A data-source neutral query language
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LINQ support in the C#/.NET driver

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• Typical na=ve query
> db.employees.find({
Name : /^John/
Salary : { $gte : 100000 }
})
{
Name : "John Doe",
Salary : 120000,
....
}
...
>
Native queries (in the mongo shell)

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• Typical na=ve query in C#
var collection = database.GetCollection("employees");
var query = Query.And(
Query.Matches("Name", "/^John/"),
Query.GTE("Salary", 100000)
);
foreach (var employee in collection.Find(query)) {
// process employee
}
Native queries (in C#)

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• LINQ is supported for queries only (although
Query.Where supports wri=ng predicates in C#
that can be used with Update)
• You opt-‐in to LINQ using AsQueryable()
Using LINQ for queries

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• Typical query using LINQ
var collection = database.GetCollection("employees");
var query =
from e in collection.AsQueryable()
where e.Name.StartsWith("John") && e.Salary >= 100000
select e;
foreach (var employee in query) {
// process employee
}
Using LINQ for queries

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• C# compiler creates an Expression tree
• C#/.NET driver translates the Expression tree to an
equivalent MongoDB query at run =me
• Requirements
– For each C# property referenced in the LINQ query the
driver has to be able to ﬁgure out the matching element
name in the BSON document (using doZed names for
nested elements)
– For each test using those C# proper=es the driver has to be
be able to translate the test into an equivalent MongoDB
query operator
What's really happening when you use LINQ?

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• The primary goal is: we will only support LINQ queries
that have a reasonable transla=on to an equivalent
MongoDB query
• The reason: we want to ensure predictable
performance from LINQ queries (no black magic, no
surprises)
• So, you will not ﬁnd:
– Any hidden map/reduce or Javascript tricks
– Any hidden client side processing
• Online LINQ tutorial at: hZp://www.mongodb.org/display/DOCS/
CSharp+Driver+LINQ+Tutorial
Design goals of the LINQ support

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var query =
where e.EmployeeStatus == Status.Active
select e;
// translates to (assuming enum value for Active is 1):
{ EmployeeStatus : 1 }
var query =
where e.EmployeeStatus != Status.Active
select e;
// translates to:
{ EmployeeStatus : { $ne : 1 } }
Sample LINQ queries (== and !=)

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var query =
where
e.EmployeeStatus == Status.Active &&
e.Salary > 100000
select e;
// translates to:
{ EmployeeStatus : 1, Salary : { $gt : 100000 } }
Sample LINQ queries (&&)

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var query =
where
e.EmployeeStatus == Status.Active ||
e.Salary > 100000
select e;
// translates to:
{ $or : [
{ EmployeeStatus : 1 },
{ Salary : { $gt : 100000 } }
]}
Sample LINQ queries (||)

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var query =
where e.Name.Contains("oh")
select e;
// translates to:
{ nm: /oh/s }
var query =
where e.Name.StartsWith("John")
select e;
// translates to:
{ nm: /^John/s }
Sample LINQ queries (string Contains, StartsWith)

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var query =
where e.Name.Length == 4
select e;
// translates to:
{ nm: /^.{4}$/s }
var query =
where string.IsNullOrEmpty(e.Name)
select e;
// translates to:
{ $or : [ { nm: { $type : 10 } }, { nm: "" } ] }
Sample LINQ queries (string Length, IsNullOrEmpty)

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var query =
where e.Name.ToLower() == "john macadam"
select e;
// translates to:
{ nm: /^john macadam$/is }
Sample LINQ queries (string ToLower)

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var query =
where e.Skills[0] == "Java"
select e;
// translates to:
{ "Skills.0" : "Java" }
var query =
where e.Skills.Length == 3
select e;
// translates to:
{ Skills : { $size : 3 } }
Sample LINQ queries (array element, Length)

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var query =
where e.Address.City == "Hoboken"
select e;
// translates to:
{ "Address.City" : "Hoboken" }
Sample LINQ queries (dotted names)

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var states = new [] { "NJ", "NY", "PA" };
var query =
where states.Contains(e.Address.State)
select e;
// translates to:
{ "Address.State" : { $in : [ "NJ", "NY", "PA" ] } }
// alternative syntax using C#/.NET driver "In" method
var query =
where e.Address.State.In(states)
select e;
Sample LINQ queries ($in)

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var desiredSkills = new [] { "Java", "C#" };
var query =
where e.Skills.ContainsAny(desiredSkills)
select e;
// translates to:
{ "Skills" : { $in : [ "Java", "C#" ] } }
var query =
where e.Skills.ContainsAll(desiredSkills)
select e;
// translates to:
{ "Skills" : { $all : [ "Java", "C#" ] } }
note: ContainsAny and ContainsAll are defined by the C#/.NET
driver and are not part of standard LINQ
Sample LINQ queries ($in/$all with arrays)

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var query =
where
e.Addresses.Any(a =>
a.City == "Hoboken" &&
a.State == "NJ")
select e;
// translates to:
{ "Addresses" : { $elemMatch :
{ City : "Hoboken", State : "NJ" } } }
Sample LINQ queries ($elemMatch)

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• You can "Inject" na=ve MongoDB queries into a LINQ
query if you need to include a test that LINQ doesn't
support, without giving up LINQ en=rely
var query =
where
e.Salary > 50000 &&
Query.NotExists("EmployeeStatus").Inject()
select e;
// translates to:
{
Salary : { $gt : 50000 },
EmployeeStatus : { $exists : false }
}
Mixing LINQ with MongoDB queries

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Open source, high performance database
Q&A
Robert Stam
Software Engineer, 10gen

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New LINQ support in C#/.NET driver Robert Stam, 10gen

New LINQ support in C#/.NET driver Robert Stam, 10gen

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