grapher/docs/reducers.md

Reducers

The reducers are a sort of "smart fields" which allow you to compose different results from your query.

To achieve this we extend Mongo.Collection with an addReducer() method:

Collection.addReducer({
    reducerName: {
        body: graphDependencyBody,
        reduce(object) {
            return value; // can be anything, object, date, string, number, etc
        }
    }
})

Basics

Meteor.users.addReducers({
    fullName: {
        body: {
            profile: {
                firstName: 1,
                lastName: 1
            }
        },
        reduce(object) {
            const {profile} = object;
            
            return `${profile.firstName} ${profileLastName}`;
        }
    }
})

Query:

const user = Meteor.users.createQuery({
    fullName: 1,
}).fetchOne();

Results to:

{
    _id: 'XXX',
    fullName: 'John Smith',
}

Reducers and links

Easily grab the data from your links (as deep as you want them), if you want to reduce it.

Meteor.users.addReducers({
    groupNames: { 
        body: {
            // assuming you have a link called groups
            groups: { name: 1 } 
        },
        reduce(object) { // a pure function that returns the data
            return object.groups.map(group => group.name).join(',')
        }
    }
})

Query:

const user = Meteor.users.createQuery({
    groupNames: 1,
}).fetchOne();

Result:

{
    _id: 'XXX',
    groupNames: ['Group 1', 'Group 2'],
}

Note that groups: [] is not present in your result set. This is because we detect the fact that you did not include it in the body of your query, however if you would have done:

Query:

const user = Meteor.users.createQuery({
    groupNames: 1,
    groups: {
        createdAt: 1,
    }
}).fetchOne();

Result:

{
    _id: 'XXX',
    groupNames: ['Group 1', 'Group 2'],
    groups: [
        {_id: 'groupId1', createdAt: Date},
        {_id: 'groupId2', createdAt: Date},
    ]
}

Notice that group name is not there. This is because we clean leftovers so the result is predictable.

Reducers and reducers

You can also use other reducers inside your reducers.

// setting up
Users.addReducers({
    fullName: {...}
    fullNameWithRoles: { // the name of how you want to request it
        body: { // the dependency, what info it needs to be able to reduce
            fullName: 1,
            roles: 1
        },
        reduce(object) { // a pure function that returns the data
            return object.fullName + object.roles.join(',');
        }
    }
})

And again, unless you specified fullName: 1 in your query, it will not be present in the result set.

Params-aware reducers

By default the reducer receives the parameters the query has.

This can open the path to some nice customizations:

Collection.addReducers({
    reducer: {
        body,
        reduce(user, params) {}
    }
})

Be aware that this reducer may be used from any queries with different types of parameters.

Reducers can do anything!

If we want to just receive the number of posts a user has, we can use reducers for this:

Meteor.users.addReducers({
    postCount: {
        body: {_id: 1},
        reduce(user) {
            const linker = Users.getLink(user, 'posts');
            
            return linker.find().count();
        }
    }
})

Or if you want to fetch some data from an external API:

Note that these reducers need to be defined server-side only, and they can only work with static queries.

Projects.addReducers({
    githubStars: {
        body: {
            repository: 1,
        },
        reduce(collectionItem) {
            const {repository} = collectionItem;
            const call = Meteor.wrapAsync(API.doSomething, API);
            // you can use anything that is in sync
            // don't return the result inside a callback because it won't work.
            call();
        },
    }
})

Filtering by reducers

If you want to filter reducers you can use $postFilters or $postFilter special functions.

Conclusion

Reducers are a neat way to remove boilerplate from your code, especially for our infamous emails[0].address, inside Meteor.users collection, check if you can figure out how to reduce it!