如何查询ActiveModel记录的子集？

Question

I've rewritten this question as my previous explanation was causing confusion.

In the SQL world, you have an initial record set that you apply a query to. The output of this query is the result set . Generally, the initial record set is an entire table of records and the result set is the records from the initial record set that match the query ruleset.

I have a use case where I need my application to occasionally operate on only a subset of records in a table. If a table has 10,000 records in it, I'd like my application to behave like only the first 1,000 records exist. These should be the same 1,000 records each time. In other words, I want the initial record set to be the first 1,000 devices in a table (when ordered by primary key), and the result set the resulting records from these first 1,000 devices.

Some solutions have been proposed, and it's revealed that my initial description was not very clear. To be more explicit, I am not trying to implement pagination. I'm also not trying to limit the number of results I receive (which .limit(1,000) would indeed achieve).

Thanks!

Answer 1

This is the line in your question that I don't understand:

This causes issues though with both of the calls, as limit limits the results of the query, not the database rows that the query is performed on.

This is not a Rails thing, this is a SQL thing.

Device.limit(n) runs SELECT * FROM device LIMIT n

Limit always returns a subset of the queried result set.

Would first(n) accomplish what you want? It will both order the result set ascending by the PK and limit the number of results returned.

Answer 2

SQL Statements can be chained together. So if you have your subset, you can then perform additional queries with it.

my_subset = Device.where(family: "Phone")
# SQL: SELECT * FROM Device WHERE `family` = "Phone"

my_results = my_subset.where(style: "Touchscreen")
# SQL: SELECT * FROM Device WHERE `family` = "Phone" AND `style` = "Touchscreen"

Which can also be written as:

my_results = Device.where(family: "Phone").where(style: "Touchscreen")
my_results = Device.where(family: "Phone", style: "Touchscreen")
# SQL: SELECT * FROM Device WHERE `family` = "Phone" AND `style` = "Touchscreen"

From your question, if you'd like to select the first 1,000 rows (ordered by primary key, pkey ) and then query against that, you'll need to do:

my_results = Device.find_by_sql("SELECT * 
                           FROM (SELECT * FROM devices ORDER BY pkey ASC LIMIT 1000) 
                           WHERE `more_searching` = 'happens here'")

Answer 3

You could specifically ask for a set of IDs:

Device.where(id: (1..4).to_a)

That will construct a WHERE clause like:

WHERE id IN (1,2,3,4)