JSON value truncate floating numbers

Bug description

When I store a JSON value with floating numbers, Prisma seems to truncate the number and this leads to loss of precision and incorrect values.

How to reproduce

await prisma.user.create({
  data: {
    color: {
      r: 0.92156863212585449,
      g: 0.34117648005485535,
      b: 0.34117648005485535,
    },
  }
})

The logged query is (which makes me think this comes from Prisma and not from the database):

{
  timestamp: 2020-10-14T11:52:11.040Z,
  query: 'INSERT INTO "public"."User" ("id","color") VALUES ($1,$2) RETURNING "public"."User"."id"',
  params: '["ckg9c3vha0035qns5jyeyy8pb",{"r":0.9215686321258544,"g":0.34117648005485535,"b":0.34117648005485535}]',
  duration: 0,
  target: 'quaint::connector::metrics'
}

Notice the ending of the r number, it’s truncated. But not for the b or g value.

Expected behavior

The stored value for r should be 0.92156863212585449 and not 0.9215686321258544.

Prisma information

model User {
  color Json
}

Environment & setup

• OS: MacOS
• Database: PostgreSQL
• Node.js version: v12.16.2
• Prisma version:

@prisma/cli          : 2.8.1
Current platform     : darwin
Query Engine         : query-engine 439da16b2f8314c6faca7d2dad2cdcf0732e8a9c (at node_modules/@prisma/cli/query-engine-darwin)
Migration Engine     : migration-engine-cli 439da16b2f8314c6faca7d2dad2cdcf0732e8a9c (at node_modules/@prisma/cli/migration-engine-darwin)
Introspection Engine : introspection-core 439da16b2f8314c6faca7d2dad2cdcf0732e8a9c (at node_modules/@prisma/cli/introspection-engine-darwin)
Format Binary        : prisma-fmt 439da16b2f8314c6faca7d2dad2cdcf0732e8a9c (at node_modules/@prisma/cli/prisma-fmt-darwin)
Studio               : 0.296.0

Related

This could be related https://github.com/prisma/prisma/issues/3479 https://github.com/prisma/prisma/issues/2903

[timsuchanek]

@Kerumen thanks for the clarification. That then indeed is a bug. The Decimal type will not fix this one, as you have to use it outside of Json. We’ll further investigate 🙏

[pimeys]

There’s a fix coming. The problem in general is how JSON doesn’t use actual f64’s to represent the numbers, so converting them to standard floats may cause accuracy errors and this is what we see here.

Luckily Rust’s JSON library has a compile time flag, that uses slower but one to one accurate conversion to and from JSON. This is enough with PostgreSQL.

Then, again, if anybody ever reads this issue, do not try to store floats in MySQL’s JSON data type:

create table foo (id int primary key, data json);
insert into foo (id, data) values (1,'0.9215686321258545');
select data from foo;

Returning 0.9215686321258544…