如何在Mongo.Net驱动程序中链接FilterDefinitionBuilders

Question

I have a filter that I would like to add to each of my queries as a basis to any of my queries that I am going to perform on the database. This will mean that i don't have to remember to add a filter ( filter.Eq("deleted", false) & filter.Eq("another query", null) on every call i make to my collection.

Using other fluent interfaces in the past i assumed that we could simply pass the builder around adding as many filters as i liked until I was actually ready to perform the query and then i would call a .Build() command and this would finally give me my FilterDefinition object.

However when I try combining the Eq() calls it simply converts to a FilterDefinition straight away rather than leaving it as a FilterDefinitionBuilder until it's time has come. This seems fairly inextensible and short sighted by the Mongo as adding a .Build() functionality would have allowed you to chain commands in a more flexible manner.

Firstly is there any reason behind the decision to do it in this manner. I want to understand why it was done like this as I know that people don't just arbitrarily make these decisions.

Secondly, has anyone managed to write any extension methods that would help me with my current predicament that they wouldn't mind sharing.

My first attempt was to include something like the following:

public static FilterDefinitionBuilder<TDocument>(this FilterDefinitionBuilder<TDocument> filter)
{
    var name = GetCollectionName<TDocument>();
    return filter.Eq("deleted", false);
}

Then i would call this to from a wrapper function that I would use instead of calling the static Builders<CosmosDocumentType>.Filter I would call a utility function that would wrap this and return a FilterDefinitionBuilder object that I can add some more filters to.

Once complete I'd like to call a .Build() method and then be presented with a rather lovely and rather functional FilterDefinition object that will get what documents I want from the collection.

Having this .Build() would also allow me to query all documents in a collection that are not deleted without having to add more filters, because at present this seems to be the only way to convert between a FilterDefinitionBuilder and a FilterDefinition .

The other alternative is writing my own Fluent wrapper around the functionality and run it through a FilterDefinitionBuilder at the end when I am satisfied that I have all of the filters in place.

If there is a way to pull the existing filters out of a FilterDefinition that might also help.

Answer 1

As I understand it, you need to decorate all your filters with a base filter . You can carry out using various approaches and I list three below.

Let's say you have the following for your base filter:

var builder = Builders<BsonDocument>.Filter;
var baseFilter = builder.Eq("deleted", false) & 
                 builder.Eq("another query", BsonNull.Value);

And the following filters that you need to decorate:

var filter01 = Builders<BsonDocument>.Filter.Eq("value", 1);
var filter02 = Builders<BsonDocument>.Filter.Eq("value", 2);

Then:

1. We can go about having a higher-order decorator function that takes a base filter and returns a function whose application on other filters will result in a filter that will be decorated with the base filter.

Its implementation is simple as follows:

public static Func<FilterDefinition<T>, FilterDefinition<T>>
  DecoratedFilter<T>(FilterDefinition<T> baseFilter) =>
    filter => baseFilter & filter;

You can now use the DecoratedFilter() function to get a function and use it on your filters as follows:

var decorate = DecoratedFilter(baseFilter);

var decoratedFilter01 = decorate(filter01);
var decoratedFilter02 = decorate(filter02);

2. Another approach is for you to have an extension method itself:

public static FilterDefinition<T>
  Decorate<T>(this FilterDefinition<T> firstFilter,
              FilterDefinition<T> secondFilter) => 
    firstFilter & secondFilter;

It can be used as follows:

var decoratedFilter01 = filter01.Decorate(baseFilter);
var decoratedFilter02 = filter02.Decorate(baseFilter);

3. You can hard-code the base filter based on your particular use case and can have an implementation as follows:

public static readonly FilterDefinition<BsonDocument> BaseFilter = ...

public static FilterDefinition<BsonDocument>
  WithBaseFilter(this FilterDefinition<BsonDocument> filter) =>
    BaseFilter & filter;

Then simply use it as:

var decoratedFilter01 = filter01.WithBaseFilter();
var decoratedFilter02 = filter02.WithBaseFilter();