Related
I'm trying to access a property of an object using a dynamic name. Is this possible?
const something = { bar: "Foobar!" };
const foo = 'bar';
something.foo; // The idea is to access something.bar, getting "Foobar!"
There are two ways to access properties of an object:
Dot notation: something.bar
Bracket notation: something['bar']
The value between the brackets can be any expression. Therefore, if the property name is stored in a variable, you have to use bracket notation:
var something = {
bar: 'foo'
};
var foo = 'bar';
// both x = something[foo] and something[foo] = x work as expected
console.log(something[foo]);
console.log(something.bar)
This is my solution:
function resolve(path, obj) {
return path.split('.').reduce(function(prev, curr) {
return prev ? prev[curr] : null
}, obj || self)
}
Usage examples:
resolve("document.body.style.width")
// or
resolve("style.width", document.body)
// or even use array indexes
// (someObject has been defined in the question)
resolve("part.0.size", someObject)
// returns null when intermediate properties are not defined:
resolve('properties.that.do.not.exist', {hello:'world'})
In javascript we can access with:
dot notation - foo.bar
square brackets - foo[someVar] or foo["string"]
But only second case allows to access properties dynamically:
var foo = { pName1 : 1, pName2 : [1, {foo : bar }, 3] , ...}
var name = "pName"
var num = 1;
foo[name + num]; // 1
// --
var a = 2;
var b = 1;
var c = "foo";
foo[name + a][b][c]; // bar
Following is an ES6 example of how you can access the property of an object using a property name that has been dynamically generated by concatenating two strings.
var suffix = " name";
var person = {
["first" + suffix]: "Nicholas",
["last" + suffix]: "Zakas"
};
console.log(person["first name"]); // "Nicholas"
console.log(person["last name"]); // "Zakas"
This is called computed property names
You can achieve this in quite a few different ways.
let foo = {
bar: 'Hello World'
};
foo.bar;
foo['bar'];
The bracket notation is specially powerful as it let's you access a property based on a variable:
let foo = {
bar: 'Hello World'
};
let prop = 'bar';
foo[prop];
This can be extended to looping over every property of an object. This can be seem redundant due to newer JavaScript constructs such as for ... of ..., but helps illustrate a use case:
let foo = {
bar: 'Hello World',
baz: 'How are you doing?',
last: 'Quite alright'
};
for (let prop in foo.getOwnPropertyNames()) {
console.log(foo[prop]);
}
Both dot and bracket notation also work as expected for nested objects:
let foo = {
bar: {
baz: 'Hello World'
}
};
foo.bar.baz;
foo['bar']['baz'];
foo.bar['baz'];
foo['bar'].baz;
Object destructuring
We could also consider object destructuring as a means to access a property in an object, but as follows:
let foo = {
bar: 'Hello World',
baz: 'How are you doing?',
last: 'Quite alright'
};
let prop = 'last';
let { bar, baz, [prop]: customName } = foo;
// bar = 'Hello World'
// baz = 'How are you doing?'
// customName = 'Quite alright'
You can do it like this using Lodash get
_.get(object, 'a[0].b.c');
UPDATED
Accessing root properties in an object is easily achieved with obj[variable], but getting nested complicates things. Not to write already written code I suggest to use lodash.get.
Example
// Accessing root property
var rootProp = 'rootPropert';
_.get(object, rootProp, defaultValue);
// Accessing nested property
var listOfNestedProperties = [var1, var2];
_.get(object, listOfNestedProperties);
Lodash get can be used in different ways, the documentation lodash.get
To access a property dynamically, simply use square brackets [] as follows:
const something = { bar: "Foobar!" };
const userInput = 'bar';
console.log(something[userInput])
The problem
There's a major gotchya in that solution! (I'm surprised other answers have not brought this up yet). Often you only want to access properties that you've put onto that object yourself, you don't want to grab inherited properties.
Here's an illustration of this issue. Here we have an innocent-looking program, but it has a subtle bug - can you spot it?
const agesOfUsers = { sam: 16, sally: 22 }
const username = prompt('Enter a username:')
if (agesOfUsers[username] !== undefined) {
console.log(`${username} is ${agesOfUsers[username]} years old`)
} else {
console.log(`${username} is not found`)
}
When prompted for a username, if you supply "toString" as a username, it'll give you the following message: "toString is function toString() { [native code] } years old". The issue is that agesOfUsers is an object, and as such, automatically inherits certain properties like .toString() from the base Object class. You can look here for a full list of properties that all objects inherit.
Solutions
Use a Map data structure instead. The stored contents of a map don't suffer from prototype issues, so they provide a clean solution to this problem.
const agesOfUsers = new Map()
agesOfUsers.set('sam', 16)
agesOfUsers.set('sally', 2)
console.log(agesOfUsers.get('sam')) // 16
Use an object with a null prototype, instead of the default prototype. You can use Object.create(null) to create such an object. This sort of object does not suffer from these prototype issues, because you've explicitly created it in a way that it does not inherit anything.
const agesOfUsers = Object.create(null)
agesOfUsers.sam = 16
agesOfUsers.sally = 22;
console.log(agesOfUsers['sam']) // 16
console.log(agesOfUsers['toString']) // undefined - toString was not inherited
You can use Object.hasOwn(yourObj, attrName) to first check if the dynamic key you wish to access is directly on the object and not inherited (learn more here). This is a relatively newer feature, so check the compatibility tables before dropping it into your code. Before Object.hasOwn(yourObj, attrName) came around, you would achieve this same effect via Object.prototype.hasOwnProperty.call(yourObj, attrName). Sometimes, you might see code using yourObj.hasOwnProperty(attrName) too, which sometimes works but it has some pitfalls that you can read about here.
// Try entering the property name "toString",
// you'll see it gets handled correctly.
const user = { name: 'sam', age: 16 }
const propName = prompt('Enter a property name:')
if (Object.hasOwn(user, propName)) {
console.log(`${propName} = ${user[propName]}`)
} else {
console.log(`${propName} is not found`)
}
If you know the key you're trying to use will never be the name of an inherited property (e.g. maybe they're numbers, or they all have the same prefix, etc), you can choose to use the original solution.
I came across a case where I thought I wanted to pass the "address" of an object property as data to another function and populate the object (with AJAX), do lookup from address array, and display in that other function. I couldn't use dot notation without doing string acrobatics so I thought an array might be nice to pass instead. I ended-up doing something different anyway, but seemed related to this post.
Here's a sample of a language file object like the one I wanted data from:
const locs = {
"audioPlayer": {
"controls": {
"start": "start",
"stop": "stop"
},
"heading": "Use controls to start and stop audio."
}
}
I wanted to be able to pass an array such as: ["audioPlayer", "controls", "stop"] to access the language text, "stop" in this case.
I created this little function that looks-up the "least specific" (first) address parameter, and reassigns the returned object to itself. Then it is ready to look-up the next-most-specific address parameter if one exists.
function getText(selectionArray, obj) {
selectionArray.forEach(key => {
obj = obj[key];
});
return obj;
}
usage:
/* returns 'stop' */
console.log(getText(["audioPlayer", "controls", "stop"], locs));
/* returns 'use controls to start and stop audio.' */
console.log(getText(["audioPlayer", "heading"], locs));
ES5 // Check Deeply Nested Variables
This simple piece of code can check for deeply nested variable / value existence without having to check each variable along the way...
var getValue = function( s, context ){
return Function.call( context || null, 'return ' + s )();
}
Ex. - a deeply nested array of objects:
a = [
{
b : [
{
a : 1,
b : [
{
c : 1,
d : 2 // we want to check for this
}
]
}
]
}
]
Instead of :
if(a && a[0] && a[0].b && a[0].b[0] && a[0].b[0].b && a[0].b[0].b[0] && a[0].b[0].b[0].d && a[0].b[0].b[0].d == 2 ) // true
We can now :
if( getValue('a[0].b[0].b[0].d') == 2 ) // true
Cheers!
Others have already mentioned 'dot' and 'square' syntaxes so I want to cover accessing functions and sending parameters in a similar fashion.
Code jsfiddle
var obj = {method:function(p1,p2,p3){console.log("method:",arguments)}}
var str = "method('p1', 'p2', 'p3');"
var match = str.match(/^\s*(\S+)\((.*)\);\s*$/);
var func = match[1]
var parameters = match[2].split(',');
for(var i = 0; i < parameters.length; ++i) {
// clean up param begninning
parameters[i] = parameters[i].replace(/^\s*['"]?/,'');
// clean up param end
parameters[i] = parameters[i].replace(/['"]?\s*$/,'');
}
obj[func](parameters); // sends parameters as array
obj[func].apply(this, parameters); // sends parameters as individual values
I asked a question that kinda duplicated on this topic a while back, and after excessive research, and seeing a lot of information missing that should be here, I feel I have something valuable to add to this older post.
Firstly I want to address that there are several ways to obtain the value of a property and store it in a dynamic Variable. The first most popular, and easiest way IMHO would be:
let properyValue = element.style['enter-a-property'];
however I rarely go this route because it doesn't work on property values assigned via style-sheets. To give you an example, I'll demonstrate with a bit of pseudo code.
let elem = document.getElementById('someDiv');
let cssProp = elem.style['width'];
Using the code example above; if the width property of the div element that was stored in the 'elem' variable was styled in a CSS style-sheet, and not styled inside of its HTML tag, you are without a doubt going to get a return value of undefined stored inside of the cssProp variable. The undefined value occurs because in-order to get the correct value, the code written inside a CSS Style-Sheet needs to be computed in-order to get the value, therefore; you must use a method that will compute the value of the property who's value lies within the style-sheet.
Henceforth the getComputedStyle() method!
function getCssProp(){
let ele = document.getElementById("test");
let cssProp = window.getComputedStyle(ele,null).getPropertyValue("width");
}
W3Schools getComputedValue Doc This gives a good example, and lets you play with it, however, this link Mozilla CSS getComputedValue doc talks about the getComputedValue function in detail, and should be read by any aspiring developer who isn't totally clear on this subject.
As a side note, the getComputedValue method only gets, it does not set. This, obviously is a major downside, however there is a method that gets from CSS style-sheets, as well as sets values, though it is not standard Javascript.
The JQuery method...
$(selector).css(property,value)
...does get, and does set. It is what I use, the only downside is you got to know JQuery, but this is honestly one of the very many good reasons that every Javascript Developer should learn JQuery, it just makes life easy, and offers methods, like this one, which is not available with standard Javascript.
Hope this helps someone!!!
For anyone looking to set the value of a nested variable, here is how to do it:
const _ = require('lodash'); //import lodash module
var object = { 'a': [{ 'b': { 'c': 3 } }] };
_.set(object, 'a[0].b.c', 4);
console.log(object.a[0].b.c);
// => 4
Documentation: https://lodash.com/docs/4.17.15#set
Also, documentation if you want to get a value: https://lodash.com/docs/4.17.15#get
You can do dynamically access the property of an object using the bracket notation. This would look like this obj[yourKey] however JavaScript objects are really not designed to dynamically updated or read. They are intended to be defined on initialisation.
In case you want to dynamically assign and access key value pairs you should use a map instead.
const yourKey = 'yourKey';
// initialise it with the value
const map1 = new Map([
['yourKey', 'yourValue']
]);
// initialise empty then dynamically assign
const map2 = new Map();
map2.set(yourKey, 'yourValue');
console.log(map1.get(yourKey));
console.log(map2.get(yourKey));
demo object example
let obj = {
name: {
first_name: "Bugs",
last_name: "Founder",
role: "Programmer"
}
}
dotted string key for getting the value of
let key = "name.first_name"
Function
const getValueByDottedKeys = (obj, strKey)=>{
let keys = strKey.split(".")
let value = obj[keys[0]];
for(let i=1;i<keys.length;i++){
value = value[keys[i]]
}
return value
}
Calling getValueByDottedKeys function
value = getValueByDottedKeys(obj, key)
console.log(value)
output
Bugs
const getValueByDottedKeys = (obj, strKey)=>{
let keys = strKey.split(".")
let value = obj[keys[0]];
for(let i=1;i<keys.length;i++){
value = value[keys[i]]
}
return value
}
let obj = {
name: {
first_name: "Bugs",
last_name: "Founder",
role: "Programmer"
}
}
let key = "name.first_name"
value = getValueByDottedKeys(obj, key)
console.log(value)
I bumped into the same problem, but the lodash module is limited when handling nested properties. I wrote a more general solution following the idea of a recursive descendent parser. This solution is available in the following Gist:
Recursive descent object dereferencing
Finding Object by reference without, strings,
Note make sure the object you pass in is cloned , i use cloneDeep from lodash for that
if object looks like
const obj = {data: ['an Object',{person: {name: {first:'nick', last:'gray'} }]
path looks like
const objectPath = ['data',1,'person',name','last']
then call below method and it will return the sub object by path given
const child = findObjectByPath(obj, objectPath)
alert( child) // alerts "last"
const findObjectByPath = (objectIn: any, path: any[]) => {
let obj = objectIn
for (let i = 0; i <= path.length - 1; i++) {
const item = path[i]
// keep going up to the next parent
obj = obj[item] // this is by reference
}
return obj
}
You can use getter in Javascript
getter Docs
Check inside the Object whether the property in question exists,
If it does not exist, take it from the window
const something = {
get: (n) => this.n || something.n || window[n]
};
You should use JSON.parse, take a look at https://www.w3schools.com/js/js_json_parse.asp
const obj = JSON.parse('{ "name":"John", "age":30, "city":"New York"}')
console.log(obj.name)
console.log(obj.age)
I'm looking to combine many IObservable<bool> streams such that when the latest value for all of them is true, a true is emitted, and otherwise a false is emitted.
CombinedLast would allow me to build something like this for two streams easily, but a) I'm not sure the API easily allows thousands of streams to be combined and b) I'm not sure how efficient it would be even if it could.
All is kinda similar to what I want except I'm assuming that works over a single sequence and once false cannot dynamically changes back to true.
Also I need the values to be "distinct until changed", although the DistintUntilChanged operator may not be efficient for this?
I'm hoping for an O(1) algorithm.
A good approach for combining the latest is to start with a IObservable<IObservable<T>> and turn it in to a IObservable<T[]>. This becomes a very dynamic way to combine as many values you need.
Here's an extension method to do this:
public static IObservable<T[]> CombineLatest<T>(this IObservable<IObservable<T>> sources)
{
return
sources.Publish(ss =>
Observable.Create<T[]>(o =>
{
var composite = new CompositeDisposable();
var list = new List<T>();
composite.Add(
ss.Subscribe(source =>
{
var index = list.Count;
list.Add(default(T));
composite.Add(source.Subscribe(x => list[index] = x));
}));
composite.Add(ss.Merge().Select(x => list.ToArray()).Subscribe(o));
return composite;
}));
}
This nicely creates and tracks all subscriptions and uses a closure to define the index that each subscription needs to use to update its value in the list that is used for output.
If you use it like this:
var sources = new Subject<IObservable<bool>>();
var output = sources.CombineLatest();
output.Subscribe(x => Console.WriteLine(x));
var s1 = new Subject<bool>();
sources.OnNext(s1);
s1.OnNext(true);
var s2 = new Subject<bool>();
sources.OnNext(s2);
s2.OnNext(false);
var s3 = new Subject<bool>();
sources.OnNext(s3);
s3.OnNext(true);
s2.OnNext(true);
s1.OnNext(false);
Then you get this output:
If you change the definition of output to var output = sources.CombineLatest().Select(xs => xs.Aggregate((x, y) => x & y)); then you get the output that I think you're after:
True
False
False
True
False
I don't know how to do this in a classically functional way and still achieve O(1). This used mutable state, and is O(1) for observing each message, but O(n) for memory:
public IObservable<bool> CombineBooleans(this IObservable<bool>[] source)
{
return source.Select((o, i) => o.Select(b => (value: b, index: i)))
.Merge()
.Scan((array: new bool[source.Length], countFalse: source.Length), (state, item) =>
{
var countFalse = state.countFalse;
if (state.array[item.index] == item.value)
return (state.array, countFalse); //nothing to change, emit same state
else if (state.array[item.index]) //previous/current state is true, becoming false
{
countFalse++;
state.array[item.index] = false;
}
else //previous/current state is false, becoming true
{
countFalse--;
state.array[item.index] = true;
}
return (state.array, countFalse);
})
.Scan((countFalse: source.Length, oldCountFalse: source.Length), (state, item) => (countFalse: item.countFalse, oldCountFalse: state.countFalse))
.SelectMany(state =>
state.countFalse == 1 && state.oldCountFalse == 0
? Observable.Return(false)
: state.countFalse == 0 && state.oldCountFalse == 1
? Observable.Return(true)
: Observable.Empty<bool>()
)
.Publish()
.RefCount();
}
EDIT: Added .Publish().Refcount() to eliminate multiple-subscriber bugs.
How can I apply constrains to list of list, similarly to what I can do to simple list:
list_size: uint;
my_list: list of uint;
keep my_list.size() == list_size;
keep for each (item) using index (item_index) in my_list { item == item_index;};
My intention is to create something like:
list_size:uint;
grosslist_size:uint;
my_grosslist: list of list of uint;
keep my_grosslist.size() == grosslist_size;
keep for each (grossitem) using index (grossindex)in my_grosslist {
grossitem.size() == list_size;
// keep for each (item) using index (item_index) in grossitem {
// item == item_index + grossindex * 100;
// };
};
How can I write 3 lines commented above using Specman syntax?
Please note that constrains are for instance only, in reality I'll need to apply much more sophisticated ones rather than indexing list items...
Thanks in advance.
The code you wrote is indeed the correct usage of list-of-list. Note that there was a missing space and the additional 'keep' is not needed for the internal for each. other than that, it works.
<'
extend sys {
list_size:uint;
grosslist_size:uint;
my_grosslist: list of list of uint;
keep my_grosslist.size() == grosslist_size;
keep for each (grossitem) using index (grossindex) in my_grosslist {
grossitem.size() == list_size;
for each (item) using index (item_index) in grossitem {
item == item_index + grossindex * 100;
};
};
};
'>
The code block below answers the question: "How do you perform a left outer join using linq extension methods?"
var qry = Foo.GroupJoin(
Bar,
foo => foo.Foo_Id,
bar => bar.Foo_Id,
(x,y) => new { Foo = x, Bars = y })
.SelectMany(
x => x.Bars.DefaultIfEmpty(),
(x,y) => new { Foo = x, Bar = y});
How do you write this GroupJoin and SelectMany as MethodCallExpressions? All of the examples that I've found are written using DynamicExpressions translating strings into lambdas (another example). I like to avoid taking a dependency on that library if possible.
Can the query above be written with Expressions and associated methods?
I know how to construct basic lambda expressions like foo => foo.Foo_Id using ParameterExpressions MemberExpressions and Expression.Lambda() , but how do you construct (x,y) => new { Foo = x, Bars = y })??? to be able to construct the necessary parameters to create both calls?
MethodCallExpression groupJoinCall =
Expression.Call(
typeof(Queryable),
"GroupJoin",
new Type[] {
typeof(Customers),
typeof(Purchases),
outerSelectorLambda.Body.Type,
resultsSelectorLambda.Body.Type
},
c.Expression,
p.Expression,
Expression.Quote(outerSelectorLambda),
Expression.Quote(innerSelectorLambda),
Expression.Quote(resultsSelectorLambda)
);
MethodCallExpression selectManyCall =
Expression.Call(typeof(Queryable),
"SelectMany", new Type[] {
groupJoinCall.ElementType,
resultType,
resultsSelectorLambda.Body.Type
}, groupJoinCall.Expression, Expression.Quote(lambda),
Expression.Quote(resultsSelectorLambda)));
Ultimately, I need to create a repeatable process that will left join n Bars to Foo. Because we have a vertical data structure, a left-joined query is required to return what is represented as Bars, to allow the user to sort Foo. The requirement is to allow the user to sort by 10 Bars, but I don't expect them to ever use more than three. I tried writing a process that chained the code in the first block above up to 10 times, but once I got passed 5 Visual Studio 2012 start to slow and around 7 it locked up.
Therefore, I'm now trying to write a method that returns the selectManyCall and calls itself recursively as many times as is requested by the user.
Based upon the query below that works in LinqPad, the process that needs to be repeated only requires manually handling the transparent identifiers in Expression objects. The query sorts returns Foos sorted by Bars (3 Bars in this case).
A side note. This process is significantly easier doing the join in the OrderBy delegate, however, the query it produces includes the T-SQL "OUTER APPLY", which isn't supported by Oracle which is required.
I'm grateful for any ideas on how to write the projection to anonymous type or any other out-of-the-box idea that may work. Thank you.
var q = Foos
.GroupJoin (
Bars,
g => g.FooID,
sv => sv.FooID,
(g, v) =>
new
{
g = g,
v = v
}
)
.SelectMany (
s => s.v.DefaultIfEmpty (),
(s, v) =>
new
{
s = s,
v = v
}
)
.GroupJoin (
Bars,
g => g.s.g.FooID,
sv => sv.FooID,
(g, v) =>
new
{
g = g,
v = v
}
)
.SelectMany (
s => s.v.DefaultIfEmpty (),
(s, v) =>
new
{
s = s,
v = v
}
)
.GroupJoin (
Bars,
g => g.s.g.s.g.FooID,
sv => sv.FooID,
(g, v) =>
new
{
g = g,
v = v
}
)
.SelectMany (
s => s.v.DefaultIfEmpty (),
(s, v) =>
new
{
s = s,
v = v
}
)
.OrderBy (a => a.s.g.s.g.v.Text)
.ThenBy (a => a.s.g.v.Text)
.ThenByDescending (a => a.v.Date)
.Select (a => a.s.g.s.g.s.g);
If you're having trouble figuring out how to generate the expressions, you could always get an assist from the compiler. What you could do is declare an lambda expression with the types you are going to query with and write the lambda. The compiler will generate the expression for you and you can examine it to see what expressions make up the expression tree.
e.g., your expression is equivalent to this using the query syntax (or you could use the method call syntax if you prefer)
Expression<Func<IQueryable<Foo>, IQueryable<Bar>, IQueryable>> expr =
(Foo, Bar) =>
from foo in Foo
join bar in Bar on foo.Foo_Id equals bar.Foo_Id into bars
from bar in bars.DefaultIfEmpty()
select new
{
Foo = foo,
Bar = bar,
};
To answer your question, you can't really generate an expression that creates an anonymous object, the actual type isn't known at compile time. You can cheat kinda by creating a dummy object and use GetType() to get its type which you could then use to create the appropriate new expression, but that's more of a dirty hack and I wouldn't recommend doing this. Doing so, you won't be able to generate strongly typed expressions since you don't know the type of the anonymous type.
e.g.,
var dummyType = new
{
foo = default(Foo),
bars = default(IQueryable<Bar>),
}.GetType();
var fooExpr = Expression.Parameter(typeof(Foo), "foo");
var barsExpr = Expression.Parameter(typeof(IQueryable<Bar>), "bars");
var fooProp = dummyType.GetProperty("foo");
var barsProp = dummyType.GetProperty("bars");
var ctor = dummyType.GetConstructor(new Type[]
{
fooProp.PropertyType,
barsProp.PropertyType,
});
var newExpr = Expression.New(
ctor,
new Expression[] { fooExpr, barsExpr },
new MemberInfo[] { fooProp, barsProp }
);
// the expression type is unknown, just some lambda
var lambda = Expression.Lambda(newExpr, fooExpr, barsExpr);
Whenever you need to generate an expression that involves an anonymous object, the right thing to do would be to create an known type and use that in place of the anonymous type. It will have limited use yes but it's a much cleaner way to handle such a situation. Then at least you'll be able to get the type at compile time.
// use this type instead of the anonymous one
public class Dummy
{
public Foo foo { get; set; }
public IQueryable<Bar> bars { get; set; }
}
var dummyType = typeof(Dummy);
var fooExpr = Expression.Parameter(typeof(Foo), "foo");
var barsExpr = Expression.Parameter(typeof(IQueryable<Bar>), "bars");
var fooProp = dummyType.GetProperty("foo");
var barsProp = dummyType.GetProperty("bars");
var ctor = dummyType.GetConstructor(Type.EmptyTypes);
var newExpr = Expression.MemberInit(
Expression.New(ctor),
Expression.Bind(fooProp, fooExpr),
Expression.Bind(barsProp, barsExpr)
);
// lambda's type is known at compile time now
var lambda = Expression.Lambda<Func<Foo, IQueryable<Bar>, Dummy>>(
newExpr,
fooExpr,
barsExpr);
Or, instead of creating and using a dummy type, you might be able to use tuples in your expressions instead.
static Expression<Func<T1, T2, Tuple<T1, T2>>> GetExpression<T1, T2>()
{
var type1 = typeof(T1);
var type2 = typeof(T2);
var tupleType = typeof(Tuple<T1, T2>);
var arg1Expr = Expression.Parameter(type1, "arg1");
var arg2Expr = Expression.Parameter(type2, "arg2");
var arg1Prop = tupleType.GetProperty("Item1");
var arg2Prop = tupleType.GetProperty("Item2");
var ctor = tupleType.GetConstructor(new Type[]
{
arg1Prop.PropertyType,
arg2Prop.PropertyType,
});
var newExpr = Expression.New(
ctor,
new Expression[] { arg1Expr, arg2Expr },
new MemberInfo[] { arg1Prop, arg2Prop }
);
// lambda's type is known at compile time now
var lambda = Expression.Lambda<Func<T1, T2, Tuple<T1, T2>>>(
newExpr,
arg1Expr,
arg2Expr);
return lambda;
}
Then to use it:
var expr = GetExpression<Foo, IQueryable<Bar>>();
*Mongo Newbie here
I have a document containing several hundred numeric fields which I need to query in combination.
var collection = _myDB.GetCollection<MyDocument>("collection");
IMongoQuery mongoQuery; // = Query.GT("field", value1).LT(value2);
foreach (MyObject queryObj in Queries)
{
// I have several hundred fields such as Height, that are in queryObj
// how do I build a "boolean" query in C#
mongoQuery = Query.GTE("Height", Convert.ToInt16(queryObj.Height * lowerbound));
}
I have several hundred fields such as Height (e.g. Width, Area, Perimeter etc.), that are in queryObj how do I build a "boolean" query in C# that combines range queries for each field in conjunction.
I have tried to use the example Query.GT("field", value1).LT(value2);, however the compiler does not accept the LT(Value) construct. In any event I need to be able to build a complex boolean query by looping through each of the numeric field values.
Thanks for helping a newbie out.
EDIT 3:
Ok, it looks like you already have code in place to build the complicated query. In that case, you just needed to fix the compiler issue. Am assuming you want to do the following (x > 20 && x < 40) && (y > 30 && y < 50) ...
var collection = _myDB.GetCollection<MyDocument>("collection");
var queries = new List<IMongoQuery>();
foreach (MyObject queryObj in Queries)
{
//I have several hundred fields such as Height, that are in queryObj
//how do I build a "boolean" query in C#
var lowerBoundQuery = Query.GTE("Height", Convert.ToInt16(queryObj.Height * lowerbound));
var upperBoundQuery = Query.LTE("Height", Convert.ToInt16(queryObj.Height * upperbound));
var query = Query.And(lowerBoundQuery, upperBoundQuery);
queries.Add(query);
}
var finalQuery = Query.And(queries);
/*
if you want to instead do an OR,
var finalQuery = Query.Or(queries);
*/
Original Answer.
var list = _myDb.GetCollection<MyDoc>("CollectionName")
.AsQueryable<MyDoc>()
.Where(x =>
x.Height > 20 &&
x.Height < 40)
.ToList();
I have tried to use the example Query.GT("field", value1).LT(value2);,
however the compiler does not accept the LT(Value) construct.
You can query MongoDB using linq, if you are using the official C# driver. That ought to solve the compiler issue I think.
The more interesting question I have in mind is, how are you going to construct that complicated boolean query?
One option is to dynamically build an Expression and then pass that to the Where
My colleague is using the following code for something similar...
public static IQueryable<T> Where<T>(this IQueryable<T> query,
string column, object value, WhereOperation operation)
{
if (string.IsNullOrEmpty(column))
return query;
ParameterExpression parameter = Expression.Parameter(query.ElementType, "p");
MemberExpression memberAccess = null;
foreach (var property in column.Split('.'))
memberAccess = MemberExpression.Property
(memberAccess ?? (parameter as Expression), property);
//change param value type
//necessary to getting bool from string
ConstantExpression filter = Expression.Constant
(
Convert.ChangeType(value, memberAccess.Type)
);
//switch operation
Expression condition = null;
LambdaExpression lambda = null;
switch (operation)
{
//equal ==
case WhereOperation.Equal:
condition = Expression.Equal(memberAccess, filter);
lambda = Expression.Lambda(condition, parameter);
break;
//not equal !=
case WhereOperation.NotEqual:
condition = Expression.NotEqual(memberAccess, filter);
lambda = Expression.Lambda(condition, parameter);
break;
//string.Contains()
case WhereOperation.Contains:
condition = Expression.Call(memberAccess,
typeof(string).GetMethod("Contains"),
Expression.Constant(value));
lambda = Expression.Lambda(condition, parameter);
break;
}
MethodCallExpression result = Expression.Call(
typeof(Queryable), "Where",
new[] { query.ElementType },
query.Expression,
lambda);
return query.Provider.CreateQuery<T>(result);
}
public enum WhereOperation
{
Equal,
NotEqual,
Contains
}
Currently it only supports == && !=, but it shouldn't be that difficult to implement >= or <= ...
You could get some hints from the Expression class: http://msdn.microsoft.com/en-us/library/system.linq.expressions.expression.aspx
EDIT:
var props = ["Height", "Weight", "Age"];
var query = _myDb.GetCollection<MyDoc>("CName").AsQueryable<MyDoc>();
foreach (var prop in props)
{
query = query.Where(prop, GetLowerLimit(queryObj, prop), WhereOperation.Between, GetUpperLimit(queryObj, prop));
}
// the above query when iterated over, will result in a where clause that joins each individual `prop\condition` with an `AND`.
// The code above will not compile. The `Where` function I wrote doesnt accept 4 parameters. You will need to implement the logic for that yourself. Though it ought to be straight forward I think...
EDIT 2:
If you don't want to use linq, you can still use Mongo Query. You will just need to craft your queries using the Query.And() and Query.Or().
// I think this might be deprecated. Please refer the release notes for the C# driver version 1.5.0
Query.And(Query.GTE("Salary", new BsonDouble(20)), Query.LTE("Salary", new BsonDouble(40)), Query.GTE("Height", new BsonDouble(20)), Query.LTE("Height", new BsonDouble(40)))
// strongly typed version
new QueryBuilder<Employee>().And(Query<Employee>.GTE(x => x.Salary, 40), Query<Employee>.LTE(x => x.Salary, 60), Query<Employee>.GTE(x => x.HourlyRateToClients, 40), Query<Employee>.LTE(x => x.HourlyRateToClients, 60))