Room Persistence: Error:Entities and Pojos must have a usable public constructor

For me all I had to do was to add a constructor to the data class with empty params sent to it like so:

@Entity(tableName = "posts")
data class JobPost(
@Ignore
@SerializedName("companyLogo")
var companyLogo: String,
@Ignore
@SerializedName("companyName")
var companyName: String,
@Ignore
@SerializedName("isAggregated")
var isAggregated: String,
@PrimaryKey(autoGenerate = false)
@SerializedName("jobID")
var jobID: String,
@Ignore
@SerializedName("jobTitle")
var jobTitle: String,
@Ignore
@SerializedName("postedOn")
var postedOn: String,
@Ignore
@SerializedName("region")
var region: String
) {
constructor() : this("","","","","","","")
}

Same bug, much stranger solution: Do not return cursor using reactivex `Maybe<Cursor>` on your Dao. `Flowable`, `Single`, and `Observable` did not work either.

Simply bite the bullet and make the reactive call outside the Dao request.
Before:

@Dao
interface MyDao{
@Query("SELECT * FROM mydao")
fun getCursorAll(): Flowable<Cursor>
}

After:

@Dao
interface MyDao{
@Query("SELECT * FROM mydao")
fun getCursorAll(): Cursor
}
Meta:

Android Studio 3.2
Build #AI-181.5540.7.32.5014246, built on September 17, 2018
JRE: 1.8.0_152-release-1136-b06 x86_64
JVM: OpenJDK 64-Bit Server VM by JetBrains s.r.o
macOS 10.12.6

Just add the below annotation to any constructor that causes the errors and add a new blank constructor.

> @Ignore

As stated in [Room Database `Entity`][1]:

> Each entity must either have a no-arg constructor or a constructor
> whose parameters match fields (based on type and name).

So adding an empty constructor and annotating your parameterized constructor with `@Ignore` will solve your problem. An example:

public class POJO {

long id;

String firstName;

@Ignore
String lastName;

public POJO() {
}

@Ignore
public POJO(String firstName, String lastName) {
this.firstName = firstName;
this.lastName = lastName;
}

// getters and setters
// ...

}

[1]:

[To see links please register here]

Don't use to dataclass,use normal class instead. This method will solve problem

With 2.1.0-alpha6, it turned out to be an invalid return type in Dao. Fixing the return type as expected fixed it.

For me, I was using 'lat' & 'long' as a variable name in the data(Entity) class for kotlin so renaming to latitude & longitude it worked.

**Not working:**

@Entity(tableName = "table_User")
data class User(@PrimaryKey var userId : Int, @ColumnInfo(name = "first_name")
var firstName: String
, @ColumnInfo(name = "last_name") var lastName: String
, @ColumnInfo(name = "password") var password: String
, @ColumnInfo(name = "dob") var dob: Long
, @ColumnInfo(name = "address") var address: String
, @ColumnInfo(name = "lat") var latitude: Double
, @ColumnInfo(name = "long") var longitude: Double) {

}

**Working:**

@Entity(tableName = "table_User")
data class User(@PrimaryKey var userId : Int, @ColumnInfo(name = "first_name")
var firstName: String
, @ColumnInfo(name = "last_name") var lastName: String
, @ColumnInfo(name = "password") var password: String
, @ColumnInfo(name = "dob") var dob: Long
, @ColumnInfo(name = "address") var address: String
, @ColumnInfo(name = "latitude") var latitude: Double
, @ColumnInfo(name = "longitude") var longitude: Double) {

}

For a variation on FutureShocked answer that implements autoGenerate:

@Entity(tableName = "movies")
data class MovieKt(
var title: String,
var overview: String,
var poster_path: String,
var backdrop_path: String,
@Ignore var release_date: String,
@Ignore var vote_average: Double,
@Ignore var isFavorite: Int
) {
@PrimaryKey(autoGenerate = true) var id : Int = 0

constructor(title: String, overview: String, poster_path: String, backdrop_path: String) {
this(id, title, overview, poster_path, backdrop_path, "", 0.0, 0)
}
}

Like it's said in the Room docs, you are **required to make an empty public constructor**. At the same time, if you want to declare other custom constructors, you must add `@Ignore` annotation.

@Entity
public class CartItem {
@PrimaryKey
public int product_id;
public int qty;

public CartItem() {
}

@Ignore
public CartItem(int product_id, int count) {
this.product_id = product_id;
this.qty = count;
}
}

It's not a problem in your case, but for others, this error can occur if you have @Ignore params in your primary constructor, i.e. Room expects to have either:

- parameterless constructor or
- constructor with all fields not marked with @Ignore

for example:

@Entity(tableName = "movies")
data class MovieKt(
@PrimaryKey
var id : Int,
var title: String,
@Ignore var overview: String)

will not work. This will:

@Entity(tableName = "movies")
data class MovieKt(
@PrimaryKey
var id : Int,
var title: String)