Typeorm creates migration that creates already existing unique constraint

[styczynski]

Issue Description

Expected Behavior

Typeorm should detect the unique constraint for column was already created and skip its creation.

Actual Behavior

Typeorm does not detect the constraint is present and tries to create it again:

error: error: relation "UQ_7acfebc1c4dab180b06c83c66b2" already exists

Steps to Reproduce

1. Clone the repo https://github.com/styczynski/express-typeorm-starter/tree/reproduce-contraints-problem
2. Run npm install
3. Run docker-compose -f ./docker-compose.yml up --abort-on-container-exit
4. Run npm run migrate:up
5. Run npm run migrate:generate NewMigration
6. Now npm run migrate:up will fail (on the new migration).

The failure message will be:

error: error: relation "UQ_7acfebc1c4dab180b06c83c66b2" already exists

The problem is that if you change the order of constraints when setting up the table Typeorm will fail to correctly detect uniqueness of columns.
The problematic part is this (part of the User.ts model):

@Unique(["id", "userMetaId"])
@Unique(["userMetaId"])
/* ... */
@OneToOne(() => UserMeta)

The OneToOne relation requires unique constraint on the userMetaId column.
However if we create ["id", "userMetaId"] constraint and then ["userMetaId"], Typeorm will catch only the first one
and decide that the constraint is composite so the column is not unique.

This means that if we edit the initial migration and change order of constraints so that ("userMetaId") precede the other one, everything will works fine.

My Environment

Dependency	Version
Operating System	MacOS Catalina 10.15.7
Node.js version	12.7.0
Typescript version	3.9.7
TypeORM version	0.2.37

Additional Context

Relevant Database Driver(s)

DB Type	Reproducible
aurora-data-api	no
aurora-data-api-pg	no
better-sqlite3	no
cockroachdb	no
cordova	no
expo	no
mongodb	no
mysql	no
nativescript	no
oracle	no
postgres	yes
react-native	no
sap	no
sqlite	no
sqlite-abstract	no
sqljs	no
sqlserver	no

Are you willing to resolve this issue by submitting a Pull Request?

• ✅ Yes, I have the time, and I know how to start.
• ✖️ Yes, I have the time, but I don't know how to start. I would need guidance.
• ✖️ No, I don’t have the time, but I can support (using donations) development.
• ✖️ No, I don’t have the time and I’m okay to wait for the community / maintainers to resolve this issue.

PR for this issue: #8157