Golang implementation of unknown number of datastore filters (need function to append filters)

I'm working on a web app where users can select different options, such as app price, number of stars, update time, etc.

The problem is that with Datastore then it's only possible to make 1 inequality filter, so I cannot say:

query = datastore.NewQuery("iOSApp").Filter("StarRatings >=", stars).Filter("MinimumAge >=", age).Filter("Price >=", price)

Instead, I added several bool/int entities, so that if I want to say StarRatings is greater than 3 (maximum is 5), then I can say:

query = datastore.NewQuery("iOSApp").Filter("Stars2Up =", false).Filter("Stars3Up =", true)

This solution seems to be working great (and I believe it's a good/fast solution, although I may be wrong)

The problem I have now is that the function that performs this query have to perform so many checks.. the code below is only checking the StarRating filter, but I also want to filter age, price, downloads, update time, etc.

var query *datastore.Query
switch stars {
case 1:
	query = datastore.NewQuery("AppStoreApp").Filter("Stars1Up =", true)
case 2:
	query = datastore.NewQuery("AppStoreApp").Filter("Stars1Up =", false).Filter("Stars2Up =", true)
case 3:
	query = datastore.NewQuery("AppStoreApp").Filter("Stars2Up =", false).Filter("Stars3Up =", true)
case 4:
	query = datastore.NewQuery("AppStoreApp").Filter("Stars3Up =", false).Filter("Stars4Up =", true)
case 5:
	query = datastore.NewQuery("AppStoreApp").Filter("Stars3Up =", false).Filter("Stars4Up =", true)
default:
	log.Println("error...")
}
var app []iOSApp
if _, err := db.client.GetAll(ctx, query, &app); err != nil {
	log.Fatalf("err: ", err)
}
return &app, nil

If I try to do this using if/else or switch cases then I will quickly end up with a thousand different if/else/switch scenarios.. so I was thinking that I could add a function that took the query and the property as input, and then as output it would return the query containing it's new filter..

For example

// query before
query = datastore.NewQuery("AppStoreApp").Filter("Stars1Up =", false).Filter("Stars2Up =", true)

query, err = appendFilter(query, updatetime) // in this case updatetime = within 6 months as an example

// query after
query = datastore.NewQuery("AppStoreApp").Filter("Stars1Up =", false).Filter("Stars2Up =", true).Filter("UpdatedWithin6Months =", true).Filter("UpdatedWithin1Year =", false)

I would greatly appreciate help with how this appendFilter function could be implemented

The Query.Filter() method returns a derivative query that will contain all filters of the query it was called on, plus the one you specify in its arguments. So if you plan to build the final query in multiple steps, you need to store its return value.

So your appendFilter() is unnecessary, you can just do:

q := datastore.NewQuery("AppStoreApp")

// Append a filter:
q = q.Filter("filter1", value1)

// Append another filter:
q = q.Filter("filter2", value2)

To optimize your "stars" filter, it may be done like this:

func applyStarFilter(q *datastore.Query, minStars int) *Query {
	if minStars > 1 {
		q = q.Filter(fmt.Sprintf("Stars%dUp =", minStars-1), false)
	}
	return q.Filter(fmt.Sprintf("Stars%dUp =", minStars), true)
}

Taking advantage of fields having enumerated values

Your UpdatedWithinXXYY fields are very similar to stars: have fixed, enumerated values, and to filter by it, you need to set 2 filter fields (one true and one false).

If you enumerate these in advance, you can automate setting those 2 fields. For example:

func applyUpdatedWithin(q *datastore.Query, updateIdx int) *Query {
	q = q.Filter(updatedValues[updateIdx], true)
	if updateIdx+1 < len(updatedValues) {
		q = q.Filter(updatedValues[updateIdx+1], false)
	}
	return q
}

var updatedValues = []string{
	"UpdatedWithin1Day =",
	"UpdatedWithin1Week =",
	"UpdatedWithin1Month =",
	"UpdatedWithin1Year =",
}

So an example to apply min 3 stars and UpdatedWithin6Months:

q := datastore.NewQuery("AppStoreApp")
q = applyStarFilter(q, 3)
q = applyUpdatedWithin(q, 2)

Relevant to your case, for planning your Datastore indices, I highly recommend to read this article:

Index Selection and Advanced Search