如何根据条件向数据存储区查询添加过滤器

Question

I am receiving parameters from a POST and based on if those parameters exist I want to alter the datastore query. Here is an example where we may or may not receive a "title" variable from the post message. If it does exist, I want to include it in the query as a filter:

q := datastore.NewQuery("book").
    Filter("author =", "DB").
    if title != nil {Filter("title =",title).}
    Order("author")

Any suggestions?

Answer 1

Optionally call another Query.Filter() method on the query if the parameter exists. Just be sure you store the return value as it may be a different, derived query.
The reason is that all query builder/modifier methods return a derived query which you can use to chain multiple operations. You don't have to do it all in one step though, you can store the intermediate result query and continue the "work" from there (e.g. add new filters). Just remember to always store the return values, and you have to use the Query returned by the last method.

Solution (candidate):

q := datastore.NewQuery("book").Filter("author =", "DB")
if title != nil {
    q = q.Filter("title =", title)
}
q = q.Order("author")

Note:

You did not mention it, but I assume title is a string. Variables of type string cannot have a nil value. The zero value of a string is the empty string "" so use that to compare/test:

q := datastore.NewQuery("book").Filter("author =", "DB")
if title != "" {
    q = q.Filter("title =", title)
}
q = q.Order("author")