Recently, I have been looking into various solutions for interacting with databases with ease in Go. My go to library for database work in Go is sqlx, this makes unmarshalling the data from the database into structs a cinch. You write out your SQL query, tag your structs using the db tag, and let sqlx handle the rest. However, the main problem I have encountered, was with idiomatic query building. This led me to investigate this problem, and jot down some of my thoughts in this post.

TL;DR First class functions are an idiomatic way of doing SQL query building in Go. Check out the repository containing some example code I wrote testing this out: https://github.com/andrewpillar/query.

GORM, Layered Complexity, and the Active Record Pattern

Most people who dip their toe into database work in Go, will most likely be pointed towords gorm for working with databases. It is a fairly fully featured ORM, that supports migrations, relations, transactions, and much more. For those who have worked with ActiveRecord, or Eloquent, GORM’s usage would be some what familiar to you.

I have used GORM briefly before, and for simple CRUD based applications this is fine. However, when it comes to more layered complexity, I find that it falls short. Assume we are building a blogging application, and we allow users to search for posts via the search query string in a URL. If this is present, we want to constrain the query with a WHERE title LIKE , otherwise we do not.

posts := make ( [ ] Post , 0 ) search := r . URL . Query ( ) . Get ( "search" ) db := gorm . Open ( "postgres" , "..." ) if search != "" { db = db . Where ( "title LIKE ?" , "%" + search + "%" ) } db . Find ( & posts )

Nothing to controversial, we simply check to see if we have a value and modify the invocation to GORM itself. However, what if we wanted to allow searching of posts after a certain date? We would need to add some more checks, first to see if the after query string is present in the URL, and if so modify the query appropriately.

posts := make ( [ ] Post , 0 ) search := r . URL . Query ( ) . Get ( "search" ) after := r . URL . Query ( ) . Get ( "after" ) db := gorm . Open ( "postgres" , "..." ) if search != "" { db = db . Where ( "title LIKE ?" , "%" + search + "%" ) } if after != "" { db = db . Where ( "created_at > ?" , after ) } db . Find ( & posts )

So we add another check to determine if the invocation should be modified. This is working fine so far, but things could start getting out of hand. Ideally what we would want is some way of extending GORM with some custom callbacks that would accept the search , and after variables regardless of their value, and defer the logic to that custom callback. GORM does support a plugin system, for writing custom callbacks, however it seems this is more suited for modifying table state upon certain operations.

As demonstrated above, I find GORM’s biggest drawback is how cumbersone it can be to do layered complexity. More often than not when writing SQL queries, you will want this. Trying to determine if you want to add a WHERE clause to a query based off of some user input, or how you should order the records.

I believe this comes down to one thing, and I made a comment about this some time ago on HN:

Personally I think an active record style ORM for Go like gorm is a poor fit for a language that doesn’t come across as inherently OOP. Going through some of the documentation for gorm, it seems to rely heavily on method chaining which for Go seems wrong considering how errors are handled in that language. In my opinion, an ORM should be as idiomatic to the language as possible.

This comment was made on a submission of the blog post To ORM or not to ORM, which I highly recommend you read. The author of the post approaches the same conclusion about GORM that I did.

Idiomatic Query Building in Go

The database/sql package in the standard library is great for interacting with databases. And sqlx is a fine extension on top of that for handling the return of data. However, this still doesn’t fully solve the problem at hand. How can we effectively build complex queries programmatically that is idiomatic to Go. Assume we were using sqlx for the same query above, what would that look like?

posts := make ( [ ] Post , 0 ) search := r . URL . Query ( ) . Get ( "search" ) after := r . URL . Query ( ) . Get ( "after" ) db := sqlx . Open ( "postgres" , "..." ) query := "SELECT * FROM posts" args := make ( [ ] interface { } , 0 ) if search != "" { query += " WHERE title LIKE ?" args = append ( args , search ) } if after != "" { if search != "" { query += " AND " } else { query += " WHERE " } query += "created_at > ?" args = append ( args , after ) } err := db . Select ( & posts , sqlx . Rebind ( query ) , args ... )

Not much better than what we did with GORM, in fact much uglier. We’re checking if search exists twice just so we can have the correct SQL grammar in place for the query, storing our arguments in an []interface{} slice, and concatenating onto a string. This too, is not as extensible, or easy to maintain. Ideally we want to be able to build up the query, and hand it off to sqlx to handle the rest. So, what would an idiomatic query builder in Go look like? Well, it would come in one of two forms in my opinion, the first being one that utilises option structs, and the other that utilises first class functions.

Let’s take a look at squirrel. This library offers the ability to build up queries, and execute them directly in a way that I find rather idiomatic to Go. Here though, we will only be focussing on the query building aspect.

With squirrel, we can implement our above logic like so.

posts := make ( [ ] Post , 0 ) search := r . URL . Query ( ) . Get ( "search" ) after := r . URL . Query ( ) . Get ( "after" ) eqs := make ( [ ] sq . Eq , 0 ) if search != "" { eqs = append ( eqs , sq . Like { "title" , "%" + search + "%" } ) } if after != "" { eqs = append ( eqs , sq . Gt { "created_at" , after } ) } q := sq . Select ( "*" ) . From ( "posts" ) for _ , eq := range eqs { q = q . Where ( eq ) } query , args , err := q . ToSql ( ) if err != nil { return } err := db . Select ( & posts , query , args ... )

This is slightly better than what we had with GORM, and miles better than the string concatenation we were doing before. However, it still comes across as slightly cumbersone to write. squirrel uses option structs for some of the clauses in an SQL query. Optional structs are common pattern in Go for APIs that aim to be highly configurable.

An API for query building in Go should fulfill both of these needs:

Idiomacy

Extensibility

How can this be achieved with Go?

First Class Functions for Query Building

Dave Cheney has written two blog posts about first class functions, based off of a post made by Rob Pike about the same topic. For those interested they can be found here:

I’d highly recommend reading the above three posts, and using the pattern they suggest when you next come to implement an API that needs to be highly configurable.

Below is an example of what this might look like for query building:

posts := make ( [ ] * Post , 0 ) db := sqlx . Open ( "postgres" , "..." ) q := Select ( Columns ( "*" ) , Table ( "posts" ) , ) err := db . Select ( & posts , q . Build ( ) , q . Args ( ) ... )

A naive example, I know. But let’s take a look at how we might implement an API like this so that it can be used for query building. First, we should implement a query struct to keep track of the query’s state whilst it’s being built.

type statement uint8 type Query struct { stmt statement table [ ] string cols [ ] string args [ ] interface { } } const ( _select statement = iota )

The above struct will keep track of the statement we’re building, whether it’s SELECT , UPDATE , INSERT , or DELETE , table being operated on, the columns we’re working with, and the arguments that will be passed to the final query. To keep this simple, let’s focus on implementing the SELECT statement for the query builder.

Next, we need to define a type that can be used for modifying the query we’re building. This is the type that would be passed numerous times as a first class function. Each time this function is called, it should return the newly modified query, if applicable.

type Option func ( q Query ) Query

We can now implement the first part of the builder, the Select function. This will begin building a query for the SELECT statement we want to build up.

func Select ( opts ... Option ) Query { q := Query { stmt : select_ , } for _ , opt := range opts { q = opt ( q ) } return q }

You should now be able to see how everything is slowly coming together, and how the UPDATE , INSERT , and DELETE statements could be trivially implemented too. Without actually implementing some options to pass to Select , the above function is fairly useless, so let’s do that.

func Columns ( cols ... string ) Option { return func ( q Query ) Query { q . cols = cols return q } } func Table ( table string ) Option { return func ( q Query ) Query { q . table = table return q } }

As you can see, we implement these first class functions in a way so that they return the underlying option function that will be called. It would be typically expected for the option function to modify the query passed to it, and for a copy to be returned.

For this to be useful for our use case of building complex queries, we ought to implement the ability to add WHERE clauses to our query. This will require having to keep track of the various WHERE clauses in the query too.

type where struct { col string op string val interface { } } type Query struct { stmt statement table [ ] string cols [ ] string wheres [ ] where args [ ] interface { } }

We define a custom type for a WHERE clause, and add a wheres property to the original Query struct. Let’s implement two types of WHERE clauses for our needs, the first being WHERE LIKE , and the other being WHERE > .

func WhereLike ( col string , val interface { } ) Option { return func ( q Query ) Query { w := where { col : col , op : "LIKE" , val : fmt . Sprintf ( "$%d" , len ( q . args ) + 1 ) , } q . wheres = append ( q . wheres , w ) q . args = append ( q . args , val ) return q } } func WhereGt ( col string , val interface { } ) Option { return func ( q Query ) Query { w := where { col : col , op : ">" , val : fmt . Sprintf ( "$%d" , len ( q . args ) + 1 ) , } q . wheres = append ( q . wheres , w ) q . args = append ( q . args , val ) return q } }

When handling the addition of a WHERE clause to a query, we appropriately handle the bindvar syntax for the underlying SQL driver, Postgres in this case, and store the actual value itself in the args slice on the query.

So, with what little we have implemented we should be able to achieve what we want in an idiomatic way.

posts := make ( [ ] Post , 0 ) search := r . URL . Query ( ) . Get ( "search" ) after := r . URL . Query ( ) . Get ( "after" ) db := sqlx . Open ( "postgres" , "..." ) opts := [ ] Option { Columns ( "*" ) , Table ( "posts" ) , } if search != "" { opts = append ( opts , WhereLike ( "title" , "%" + search + "%" ) ) } if after != "" { opts = append ( opts , WhereGt ( "created_at" , after ) ) } q := Select ( opts ... ) err := db . Select ( & posts , q . Build ( ) , q . Args ( ) ... )

Slightly better, but still not great. However, we can extend the functionality to get what we want. So, let’s implement some functions that will return options for our specific needs.

func Search ( col , val string ) Option { return func ( q Query ) Query { if val == "" { return q } return WhereLike ( col , "%" + val + "%" ) ( q ) } } func After ( val string ) Option { return func ( q Query ) Query { if val == "" { return q } return WhereGt ( "created_at" , val ) ( q ) } }

With the above two functions implemented we can now cleanly build up a somewhat complex query for our use case. Both of these functions will only modify the query, if the values passed to them are considered correct.

posts := make ( [ ] Post , 0 ) search := r . URL . Query ( ) . Get ( "search" ) after := r . URL . Query ( ) . Get ( "after" ) db := sqlx . Open ( "postgres" , "..." ) q := Select ( Columns ( "*" ) , Table ( "posts" ) , Search ( "title" , search ) , After ( after ) , ) err := db . Select ( & posts , q . Build ( ) , q . Args ( ) ... )

I find this to be a rather idiomatic way of building up complex queries in Go. Now, of course you’ve made it this far in the post, and must be wondering, “That’s good and all, but you didn’t imeplement the Build(), or Args() methods”. This is true, to an extent. In the interest of not wanting to prolong this post any further than needed, I didn’t bother. So, if you are interested in some of the ideas presented here, take a look at the code, I submitted to GitHub. It’s nothing to rigorous, and doesn’t cover everything a query builder would need, it’s lacking JOIN , for example and supports only the Postgres bindvar.

If you have any disagreements with what I have said in this post, or would like to discuss this further, then please reach out to me at me@andrewpillar.com.