class Post extends Model {
  static table = 'posts'
  static associations = {
    comments: { type: 'has_many', foreignKey: 'post_id' },
Typescript complains:
Class static side 'typeof Post' incorrectly extends base class static side 'typeof Model'.
  Types of property 'associations' are incompatible.
    Type '{ comments: { type: string; foreignKey: string; }; }' is not assignable to type 'Associations'.
      Property 'comments' is incompatible with index signature.
        Type '{ type: string; foreignKey: string; }' is not assignable to type 'AssociationInfo'.
          Type '{ type: string; foreignKey: string; }' is not assignable to type 'HasManyAssociation'.
            Types of property 'type' are incompatible.
              Type 'string' is not assignable to type '"has_many"'.
Is there a way to solve this?
          
The solutions was declare associations as const
class Post extends Model {
  static table = 'posts'
  static associations = {
    comments: { type: 'has_many', foreignKey: 'post_id' },
  } as const  // <- here
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I'll try to elaborate, as I don't know exactly what's going wrong myself.
AFAIK, the error complains that the types don't match, specially because the object property (comments) and the information in type (has_many) and foreignKey (post_id) aren't found within the typeseting of Post or Model type. There's an issue here that I'll address later on this text.
What as const does it: instead of saying "type is receiving a value type string", it turns the strings into constants. So it says: "type is receiving a value type has_many". As I say the whole object is as const, the program understands that all the object is a constant and valid.
If anyone could elaborate this better (and even maybe say I got all this wrong), I'd be happy.
The main issue (highlighted above) is that I believe the typesetting of Watermelon isn't very good. It throws errors here and there and the typescript documentation is a single .ts file without real implementation. I do not have enough knowledge to PR a new example, so all I can do is file issues 😢
          
It's because of literal types (eg. 'belongs_to' or 'has_many') being 'widened' to type string for field type

You can also use the new 'const' assertion from typescript 3.4
class Post extends Model {
  static table = 'posts'
  static associations = {
    comments: { type: <const> 'has_many', foreignKey: 'post_id' },